What does the word heart refer to in the the Book of Proverbs Chapter 3 verse 5 in the Holy Bible King James Version?

Answer 1

The heart referred to here is the innermost part of man.

One reference book defines it this way: "The central part in general, the inside, and so for the interior man as manifesting himself in all his various activities, in his desires, affections, emotions, passions, purposes, his thoughts, perceptions, imaginations, his wisdom, knowledge, skill, his beliefs and his reasonings, his memory and his consciousness." - Journal of the Society of Biblical Literature and Exegesis, 1882, p. 67.

Other Scriptures can help add to The Bible's definition:
Psalms 17:3 (NIV) "Though you probe my heart, though you examine me at night and test me, you will find that I have planned no evil; my mouth has not transgressed."

Proverbs 17:3 (KJV) "The fining pot is for silver, and the furnace for gold: but the LORD trieth the hearts."

Proverbs 24:12 (KJV) "If thou sayest, Behold, we knew it not; doth not he that pondereth the heart consider it? and he that keepeth thy soul, doth not he know it? and shall not he render to every man according to his works?"

Matthew 9:4 (Jesus' words to the Jewish scribes who were calling him blasphemous - NLT) "Jesus knew what they were thinking, so he asked them, "Why do you have such evil thoughts in your hearts?"

Hebrews 4:12 (KJV) "For the word of God is quick, and powerful, and sharper than any two-edged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart."

Answer 2

It refers to one's mind and feelings (emotions). Trusting in God (Proverbs 3:5) should be done with both feeling and logic.