the heart: the hidden center of the whole inner person, where a human being thinks, decides, feels, and stands before God.
In the Bible the heart is not mainly the place where you feel things, the way we use the word today. It is the control room of the whole inner life: where you think and understand, where you make up your mind and set your purpose, where you feel joy or grief, and above all where you are truly yourself before God. When the Bible wants to speak of the real you, the part that no one else sees, it says "heart."
Because it covers all of that at once, the same word can lean toward any one of those facets depending on the sentence. In one place the heart reasons and ponders, in another it decides, in another it breaks or rejoices, and in another it believes or grows hard toward God. These are not different words; they are the same inner center seen from different angles, which is why English needs "mind," "will," or "feelings" in different verses where the Greek simply says "heart."
Its deepest use is moral and spiritual. The heart is what God examines and what God renews. Purity, faith, repentance, and stubbornness are all located there, because the heart is where a person says yes or no to God in the place no one but God can see. This is the thread that ties Jesus' teaching (it is from the heart that defilement comes) to Paul's (with the heart one believes) and runs straight back to the Hebrew Scriptures.