log meaning, definition, what is log: a thick piece of tree trunk or branch, especially one cut for burning on a fire. Learn more. Log in/on Meaning in the · English · Log cabin · Français. Define log: a usually bulky piece or length of a cut or fallen tree especially: a length of a tree trunk ready for sawing and — log in a sentence. Define log. log synonyms, log pronunciation, log translation, English dictionary definition of log. n. 1. a. A usually large section of a trunk or limb of a fallen or.