Jealousy is an emotion experienced by one who perceives that attention, love or affection is being given to a third party when it should rightfully be given to them. For example, a jealous lover might dislike their partner spending time with other members of the opposite sex, because they feel that they are the rightful sole recepient of their mate's attention.
The word stems from the French jalousie, formed from jaloux (jealous), and further from Low Latin zelosus (full of zeal), and from the Greek word for "ardour, zeal", with a root connoting "to boil, ferment"; or "yeast "), originally a condition of zealous emulation.
The jealousy of God, as in Exodus xx. 5, "For I, the Lord thy God, am a jealous God," has been defined by Pusey (Minor Prophets, 1860) as the attribute "whereby he does not endure the love of his creatures to be transferred from him".
"Jealous", by etymology, is however, only another form of "zealous", and the identity is exemplified by such expressions as "I have been very jealous for the Lord God of Hosts" (i Kings xix. 10).
A kind of glass, thick, ribbed and non-transparent, was formerly known as "jealous-glass," and this application is seen in the borrowed French word jalousie, a blind or shutter, made of slats of wood, which slope in such a way as to admit air and a certain amount of light, while excluding rain and sun and inspection from without.