Give the date of publication for the edition of Webster's Dictionary you are using and the page number on which your definition is found to complete your in-text citation. The date should follow your mention of Webster's Dictionary, which APA style calls the "signal phrase," and the page number should come after the quoted definition. Enclose both in parentheses. For example:
Webster's Practical Dictionary (1910) defines acorn as "the seed or fruit of an oak" (p. 5).
Begin your reference list entry with the name of the specific edition of Webster's you are using in italics followed by a period.
Webster's Practical Dictionary.
Write the year in which your edition was published in parenthesis followed by a period.
Webster's Practical Dictionary. (1910).
Conclude your entry with your edition's place and date of publication. Write the city and state of publication followed by a colon and the name of the publisher.
Webster's Practical Dictionary. (1910). Chicago, IL: G.C. Merriam.