Charles Spurgeon writes:
Repentance is a discovery of the evil of sin, a mourning that we have committed it, a resolution to forsake it. It is, in fact, a change of mind of a very deep and practical character, which makes the man love what once he hated, and hate what once he loved.
J. I. Packer writes:
Repentance means turning from as much as you know of your sin to give as much as you know of yourself to as much as you know of your God, and as our knowledge grows at these three points so our practice of repentance has to be enlarged.
John Piper writes:
Repenting means experiencing a change of mind that now sees God as true and beautiful and worthy of all our praise and all our obedience.
For further reflection, see John Piper's article titled "Thoughts on Jesus' Demand to Repent."