The word is often brushed over lightly. Here is an easy to follow article with Hebrew and Greek spelled out for you in English.
Its basic meaning is a radical and profound change of course and direction from sin and self-rule towards God. You were going in one direction, but now you are going in the opposite direction. 180 degree turn. The whole person, body (behavior), soul and spirit, must go through this change that only God initiates and can work in you.
The Old Testament was written in Hebrew, and here are three verbs indicating repenting or turning.
1.. One verb is naḥam (pronounced nah-khahm, the ḥ is pronounced like -ch- in Bach) (used 108 times). It has two distinct meanings: “to comfort, console” and “to relent, repent, change one’s mind, be grieved.” These latter definitions can sometimes apply to God’s seeming change of mind. However, this change is never a surprise or unanticipated. Rather, it means that he has conditions attached to his promises or decrees, whether stated or implied. For example, he promises something positive, and people obey, and the promise of blessing to them is fulfilled. That is his attribute of goodness. However, if they do not obey, then his attribute of justice-judgment comes forth, and he sends a prophet to warn them. If they do not repent, he judges them. If they do repent, he relents. Therefore, God is not fickle, as humans are. He simply gauges how people behave, and then he acts accordingly.
But let’s return to the topic of repentance.
2.. Another verb is ‘anah (used 79 times), and in some contexts denotes humbling in a positive sense (repentance or contrition). Some context of ‘anah pleases God (Lev. 16:31; 23:27; Ps. 35:51).
3.. The verb shub (pronounced shoob) (used 1075 times) and basically means “to turn, return, repent, go or come back.” It mostly has a physical meaning, like turning back to a point of departure. But theologically it means the act of repentance, for example, turning from idolatry to the living God. “Return to me” (Is. 44:22).
1.. The verb metanoia (pronounced meh-tah-noi-ah) (used 34 times), and it is derived from meta– (after) and nous (mind); when the prefix meta is attached to the verb as here, it means a “change of place or condition.” In this case it denotes a radical and profound moral turn of the whole person from sin to God. Repentance goes together with the forgiveness of sins (Luke 3:3; 24:7). Sins are blotted out in repentance (Acts 3:19; 5:31; 11:18; 21:21).
The Hebrew Old Testament was translated into Greek in the third century B.C., which is called the Septuagint (abbreviated LXX, for seventy translators and pronounced sep-too-ah-gent). When the verb metanoeō (see below) appears, it usually means “change one’s mind” (Prov. 20:25; Jer. 4:28). Further, the Greek verb in the LXX that often means “to turn back or return” to God is epistrephō (pronounced eh-pee-streh-foh). Behind this concept is the covenant, and Israel departing from it or returning to it (Jer. 18:8). The stem streph– means a “turning.”
2.. In the NT, the verb metanoeō (pronounced meh-tah-no-eh-oh) is the one that the writers chose instead of epistrephō, but when they appear together, metanoeō means to turn from sin, while epitstrephō means to turn to God (Acts 3:19; 26:20).
When metanoeō is used, it means a radical turn from sin to a new way of life. “Repent of your wickedness!” says Peter to Simon the sorcerer (Acts 8:22). True repentance brings forth fruit or visible evidence showing repentance (Matt. 3:8 Acts 26:20).
Then one has to believe as one changes course. He has to believe in the gospel (Mark 1:15) or the atoning death resurrection (Acts 2:38). In fact, it his faith in Christ that enables him to repent or change course.
This turning is also called “conversion,” from the Latin noun conversio, literally meaning “turning around,” which is related to the verb converto, “to turn around, to whirl around,” especially “to turn in the opposite direction” or “to turn back” or “to direct towards.”
William Mounce summarizes metanoia and metanoeō: it is “essentially to grasp the gospel message, because it does not allow for someone to obtain salvation simply by intellectually believing that Jesus is the Son of God without repenting of sins and turning to live for him” (p. 581).
So intellectual assent or agreement or changing one’s mind is insufficient. It must be a profound moral and radical change of one’s whole life.
At that link, look for Mounce.