A dictionary definition can be an important element in a research paper. According to the "MLA Handbook for Writers of Research Papers" (7th edition), properly citing borrowed words and ideas, including those from dictionaries, is necessary in order to avoid plagiarism, which can have serious consequences, and to lend credibility to your writing overall.
The Works Cited entry for a dictionary definition follows similar guidelines to that of a piece in an anthology. The entry begins with the word in quotation marks with a period. If the word has multiple entries, explain which one you quoted next by typing "Entry" (without the quotation marks) and the number. The name of the dictionary appears next, italicized and followed by a period. The book's edition is given next, along with the date and the medium of publication, such as "Print" (without the quotation marks), with periods between. For example: "Slime." Entry 2. Webster's Dictionary (italicized). 33rd ed. 2008. Print. If you use a Web-based dictionary, change the medium to "Web" and give the access date in day-month-date format with a period before and after: Web. 3 March 2013.
The parenthetical citation for a dictionary entry follows similar guidelines to that for citing a work with no author. In the in-text citation, give the word in quotation marks: ("Slime"). If the entry contains numerous definitions, the citation should identify which portion you quote. Designate this information by adding "def." (without the quotation marks) and the number and letter of the definition. Put a comma after the entry word if you add this information: ("Slime," def. 2c).