What is the biblical definition of the “heart”?

In Scripture the "heart" refers to the inner man—the place where thinking takes place. Jeremiah says this heart is deceitful, the Hebrew aqob meaning fraudulent and polluted; the mind without faith is under the dictates of the sin nature and absolutely cannot be trusted, and it is even anash, incurable and malignant. No one can understand the heart of man but God, who alone knows our real inner workings; without the convicting, teaching, and illuminating ministries of the Holy Spirit we could never understand who we are or why we do what we do.