a guide: one who goes ahead to show the way, whether leading travelers along a road or leading others in how they should live
The picture behind this word is simple: someone who walks in front and shows the way. In everyday Greek it named a caravan scout, a river pilot, or a local hired to take travelers across ground they did not know. From that plain starting point it was easily turned into a picture for teaching, since a teacher (διδάσκαλος), too, leads people down a path they cannot find on their own.
The New Testament almost always uses the word in that second, figurative way, and it does so with a sharp edge. Jesus calls the religious experts of his day "blind guides," men who have taken charge of leading others in the way of God but cannot see the road themselves. Paul quotes the same proud self-description, "a guide to the blind," and then turns it into an accusation: the one who claims to lead others has not learned to follow.
Only once does the word keep its plain, physical meaning, and even there it stings. Judas "became a guide" to the men who came to arrest Jesus, leading them through the dark to the place he knew. The one plainly literal guide in these pages leads people not along a safe road but into a betrayal.
The related verb, to guide (ὁδηγέω), tells the other half of the story. In the New Testament the ones who truly guide are the Spirit and the risen Lamb, never the human experts who claimed the title for themselves.