log·in (lôg??n?, l?g?-) also log·on (-?n?, -ôn?)n.1. The process of identifying oneself to a computer, usually by entering one’s username and password.2. A username.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.