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late 14c., from Late Latin zelus "zeal, emulation" (source of Old French zel, Italian zelo, Spanish celo), a Church word, from Greek zelos "zeal, ardor, jealousy," which is of uncertain origin.
an earnest temper; may be enlightened (Num. 25:11-13; 2 Cor. 7:11; 9:2), or ignorant and misdirected (Rom. 10:2; Phil. 3:6). As a Christian grace, it must be grounded on right principles and directed to right ends (Gal. 4:18). It is sometimes ascribed to God (2 Kings 19:31; Isa. 9:7; 37:32; Ezek. 5:13).