(1.) repent; this is a plank after shipwreck. "Let the sense of this horrid guilt which you have brought upon yourselves by putting Christ to death awaken you to a penitent reflection upon all your other sins (as the demand of some one great debt brings to light all the debts of a poor bankrupt) and to bitter remorse and sorrow for them" This was the same duty that John the Baptist and Christ had preached, and now that the Spirit is poured out is it still insisted on: "Repent, repent; change your mind, change your way; admit an after-thought."
 (2.) be baptized every one of you in the name of Jesus Christ; that is, "firmly believe the doctrine of Christ, and submit to his grace and government; and make an open solemn profession of this, and come under an engagement to abide by it, by submitting to the ordinance of baptism; be proselyted to Christ and to his holy religion, and renounce your infidelity." They must be baptized in the name of Jesus Christ. They did believe in the Father and the Holy Ghost speaking by the prophets; but they must also believe in the name of Jesus, that he is the Christ, the Messias promised to the fathers. "Take Jesus for your king, and by baptism swear allegiance to him; take him for your prophet, and hear him; take him for your priest, to make atonement for you," which seems peculiarly intended here; for they must be baptized in his name for the remission of sins upon the score of his righteousness.
(from Matthew Henry's Commentary)

Acts 17:22-31  PP34

 IV. He proceeds to call them all to repent of their idolatries, and to turn from them, v. 30, 31. This is the practical part of Paul's sermon before the university; having declared God to them (v. 23), he properly presses upon them repentance towards God, and would also have taught them faith towards our Lord Jesus Christ, if they had had the patience to hear him. Having shown them the absurdity of their worshipping other gods, he persuades them to go on no longer in that foolish way of worship, but to return from it to the living and true God.

from 3326 and 3539; to think differently or afterwards, i.e. reconsider (morally, feel compunction): 

KJV-- repent.