When using a specific definition in an essay, you should cite the dictionary on the "Works Cited" or "References" page. Each style guide has its own citation standards, which vary depending on whether the dictionary is a print source or an online source.
Steps
Method 1 of 6: Cite a printed dictionary in MLA format

Step 1. Indicate the word you defined
The word must be capitalized and enclosed in quotation marks. Then put a point. For example, if you are going to quote the word “quote”, it should appear as follows:
"Appointment"

Step 2. Enter the number of the definition
If the term has more than one dictionary definition, indicate the definition you used. A number indicates the entry number, since some words have more than one entry, and a letter indicates the definition used in that entry number. End the line with a period. Follow the format below using the “quote” example:
"Appointment". Def. 1e

Step 3. Identify the dictionary you used to define the word
Italicize the name of the dictionary, then place a period.
"Appointment". Def. 1e. Merriam-Webster's Collegiate Dictionary

Step 4. Write the year of publication
The full publication date is not required. It is only necessary to indicate the year in which the version of the dictionary you used was published, and then place a period.
"Appointment". Def. 1e. Merriam-Webster's Collegiate Dictionary. 2003

Step 5. Specify that the dictionary was printed
Since fonts can be on multiple media, the MLA style requires that you specify the medium used. In this case, it would simply be "Printed".
"Appointment". Def. 1e. Merriam-Webster's Collegiate Dictionary. 2003. Printed
Method 2 of 6: Cite an Online Dictionary in MLA Format

Step 1. Identify the word cited
Capitalize the word and enclose the word in quotation marks, and then place a period. Follow the format below using the “quote” example:
"Appointment"

Step 2. Indicate the original source
Online dictionaries often provide definitions from published print dictionaries. The dictionary, where you got the definitions, is indicated at the bottom of the entry. Italicize the name of this original dictionary and then place a period.
- "Appointment". Random House Dictionary.
- Note: If an online dictionary is an original source instead of a standalone, you can go directly to step 4, where the publication source is indicated.

Step 3. Identify the location, publisher, and year of original publication
For a publisher in a big city like New York or London, you only need to type the name of the city. However, if it is a US city that is not well known, include the State. After the location, put a colon and the name of the original publisher. Then put a comma and the year of the original dictionary publication.
"Appointment". Random House Dictionary. New York: Random House, Inc., 2012

Step 4. Specify the online publication source
The source for online publication is the online dictionary, from which you got the definition. You only need to indicate the name of the online dictionary, not the URL.
"Appointment". Random House Dictionary. New York: Random House, Inc., 2012. Dictionary.com

Step 5. Indicate that you took the definition from the web
The MLA format requires you to indicate the type of media a specific source came from.
"Appointment". Random House Dictionary. New York: Random House, Inc., 2012. Dictionary.com. Web

Step 6. Finish with the date you accessed the definition
Include the day, month, and year. You don't need to write the date in any special way, but it should end with a period.
"Appointment". Random House Dictionary. New York: Random House, Inc., 2012. Dictionary.com. Web. December 5, 2012
Method 3 of 6: Cite a printed dictionary in APA format

Step 1. Indicate the dictionary entry used
It is not necessary to put the word in quotation marks, but it is necessary to put a period after it. Follow the format below using the “quote” example:
Appointment

Step 2. Indicate the date of publication of the dictionary
The publication date of the version of the dictionary you are using must be indicated in parentheses, with a period after the closing parenthesis.
Appointment. (2003)

Step 3. Specify the name of the publisher, if available
Often this information is not given or is not known. If you don't have it, this space can be left blank.

Step 4. The name of the dictionary used
Italicize the dictionary name, but don't put any punctuation marks after it.
Appointment. (2003). Merriam-Webster's Collegiate Dictionary

Step 5. List page numbers, edition, and volume in parentheses
The page number must be indicated with "p". The edition must be specified by adding "ed." at the end, and the volume should be indicated with “vol.”. Each piece of information must be separated by a comma.
Appointment. (2003). Merriam-Webster's Collegiate Dictionary (p. 57, 11th ed., Vol. 1)

Step 6. Finish by writing the publishing location and publisher
If the name of the city is not well known or obvious, clarify where it is, including the name of the state as well. The location and publisher name must be separated by a comma, and the entire line must end with a period.
Appointment. (2003). Merriam-Webster's Collegiate Dictionary (p. 57, 11th ed., Vol. 1). Springfield, Massachusetts: Encyclopedia Britannica
Method 4 of 6: Cite an Online Dictionary in APA Format

Step 1. Enter as much information from the original post as possible
This includes the defined word, the year of publication, the original dictionary the word came from, the location of the publisher, and the publisher's name. Follow the format below using the “quote” example:
Appointment. (2012). Random House Dictionary. New York: Random House, Inc

Step 2. Indicate the source of the online dictionary from which you got the definition
You should only include the name of the web page here, and you should write it in italics.
Appointment. (2012). Random House Dictionary. New York: Random House, Inc. Dictionary.com

Step 3. Indicate the date you retrieved the definition
Include the day, month, and year. Indicate it by writing "Recovered", and place a comma after the year.
Appointment. (2012). Random House Dictionary. New York: Random House, Inc. Dictionary.com. Retrieved on December 5, 2012,

Step 4. Finish with the definition URL
Indicate the URL with the word “from”. Don't put a period at the end.
Appointment. (2012). Random House Dictionary. New York: Random House, Inc. Dictionary.com. Retrieved on December 5, 2012, from
Method 5 of 6: Cite a printed dictionary in Chicago format

Step 1. Write the name and edition of the dictionary used
You must put the name in italics and then a comma.
Merriam-Webster's Collegiate Dictionary,

Step 2. Indicates the edition of the dictionary used
The edition must be specified by following the edition number with the abbreviation “ed.”. Write a comma after the edition number.
Merriam-Webster's Collegiate Dictionary, 11th ed.,

Step 3. Indicate the word you defined
Indicate the word by writing the initials “s. v.”, which in Latin means“sub voce”, which means“under the word”. Do not capitalize the word unless it is a proper noun, and enclose the word in quotation marks. End with a period. Follow the format below using the “quote” example:
Merriam-Webster's Collegiate Dictionary, 11th ed., S. v. "appointment."
Method 6 of 6: Cite an Online Dictionary in Chicago Format

Step 1. Enter the name of the online dictionary
Italicize the name of the dictionary. Only the name of the online dictionary is required, and not the name of the original dictionary. Put a comma after the name.
Dictionary.com,

Step 2. Indicate the word you are defining
You write. v. " before the word to indicate it. In Latin, "s. V." means “sub voce”, or “under the word” in Spanish. Do not capitalize the word, but write it in quotation marks and place a comma after it. Follow the format below using the “citation” example:
Dictionary.com, s. v., "quote,"

Step 3. Indicate when you accessed the information
Indicate the information with the words “last access”. Include the month, day, and year. Insert another comma.
Dictionary.com, s. v., "appointment," last access: December 1, 2012,

Step 4. Finish with the URL
Provide the URL without any special introduction. End with a period at the end.