John preached repentance...
Our definition of what the Greek word means comes from thousands of manuscripts, both secular and Biblical, that tell us what repentance means.
Just like the English dictionary. We form definitions of words based upon how they are used in culture. Sometimes the definitions change.
For instance, "gentlemen" used to be a person who had a coat of arms and owned land.
Over time, the usage of it changed and now includes "man of refinement"... It is simply used as the culture of the day used it.
So from many sources, we know what repentance means.
If you go and look through any good Greek dictionary (Strongs is generally good) it'll even tell you the slight variations of usages we've seen.
The word used by Christ in Matthew 3 was metanoeó, which means "repent" or "change of mind"... This should tell you that repentance is a call to change. Change what??? Well, context should tell you... Repent from your sinful ways.
There is a difference between repentance and conviction as well.
Conviction is the feel that you need repentance.
"Oh, man, I feel badly about this lust" <- Conviction
"I will turn from my lust and come under the commands of Christ" <- Repentance
Conviction leads to repentance.