Probably the easiest and most direct is to say “According to an online dictionary found at http:..” and provide the rest of the reference. Footnotes are acceptable, but it doesn’t sound like your application is sufficiently formal. If you have allowed yourself time, ask your professor where to find the appropriate form. He’ll likely be glad you bothered to check with him, and tell you how to do it or tell you how to find out how to do it to his satisfaction. Remember that instructors are there to teach you, not to trick you, so find out how the professors want it done. It is entirely possible that different instructors might expect different reference styles, in spite of all the “MLA” type guidelines available. MLA is Modern Language Association used mostly for language arts. Other specialties use other forms. If the instructor has specified a set of guidelines, USE the guidelines.