An earnest temper; may be enlightened (Numbers 25:11-13; 2 Corinthians 7:11; 9:2), or ignorant and misdirected (Romans 10:2; Philippians 3:6). As a Christian grace, it must be grounded on right principles and directed to right ends (Galatians 4:18). It is sometimes ascribed to God (2 Kings 19:31; Isaiah 9:7; 37:32; Ezek. 5:13).

Noah Webster's New International Dictionary of the English Language

1. (n.) Passionate ardor in the pursuit of anything; eagerness in favor of a person or cause; ardent and active interest; engagedness; enthusiasm; fervor.

2. (n.) A zealot.

3. (v. i.) To be zealous.