The Hebrew word for jealousy is kana. (This word is a homonym of the word discussed in the essay on fasting that means "subdue" or "bring low". They are spelled very differently.)
The modern English comcept of "jealousy" has two different meanings. In ancient Hebrew the same two meanings were present.
The first use of "jealousy" is when a person wants to take what someone else has. In English this usually is phrased "jealous of..." Because this includes the yearning to take and own the desired object it is a more serious problem than merely coveting.
(A person who covets can be merely discontent while inappropriately wanting what belongs to someone else. In contrast, envy is when covetousness "flares up" and causes wrong behavior, usually some impulsive or poorly thought out rush to take or own the thing coveted. Even the very private sin of covetousness is so hurtful to relationships between people and with God that it is included in the Ten Commandments.)
This first use of "jealousy" is a new part of the English language. Previously the word envy was used to mean "I want what you have".
There are many scriptural examples of envy. In Genesis 26:14 the Philistines are envious of Isaac's wealth and can no longer bear to live near him peaceably. In Genesis 30:1 Rachel is envious of Leah's children and can no longer live in the family peaceably.
The second use of jealousy is when a person is prompted to be appreciative and protective of what he or she already has. In English this more historically proper use of the word is usually phrased "jealous over..."
This type of jealousy is not always sinful. Consider how a parent or spouse might feel when his or her child or spouse smiles at a stranger: it might cause a healthy appreciation of how much the loved one is valued and a healthy desire to remain a loved parent or spouse who receives many more smiles than a stranger.
Examples of this second kind of jealousy is found in Numbers 5:14 and 5:30, when a husband is jealous over a potentially unfaithful wife, and in Numbers 11:29, when Joshua is jealous over his teacher's status.
The concept of jealousy had not changed by the first century (which is unsurprising, as we have noted the dynamic of two related meanings continues even into modern English).
For example, Paul writes of both types of jealousy: Acts 20:33 has first kind (he has envied no one else's wealth) and Romans 10:9 and 11:11 has the second kind (Gentiles with an intimate relationship with God will provoke among faithless Jews an awareness and valuing of an intimacy with God that should already be part of their lives).
The most interesting scriptural lesson about jealousy happens in Numbers 25:10-13 when God praises Phinehas.
And said Adonai to Moses, saying, "Phinehas, son of Eleazar son of Aharon, the priest, made turn my anger from against the children of Israel in his jealousy to my jealousy among them. And I won't end the children of Israel in my jealousy." Therefore say, "Behold me, I am making to him a covenant of peace and there will be to him and his descendents after him a covenant of priesthood forever because that he was jealous for his God and made atonement over the children of Israel."
In other words, Phinehas knew Adonai was jealous over the Israelites. He understood how Adonai valued them and wanted to protect them from evil. Adonai had proclaimed earlier that he felt this second kind of jealousy over the Israelites (Exodus 20:5, 34:14). Phinehas understood with an unusual empathy.
In secular jargon we talk about people who have "a burden" for other people. Some have "a burden for the poor". Some have "a burden for the homeless". Some have a burden for those hurt by a certain kind of crime. By this we describe empathizing with someone else's distress so much we must act.
Phinehas had a burden for Adonai. He empathized with Adonai so much he was driven to action. In response, Adonai decreed that someone with a burden for Adonai is prime priest material, the most deserving (let this lineage among Aharon's sons be eternal!) among those qualified for the priesthood. Why? Because Phinehas's action, motived by a burden for Adonai, counted as atonement! This sounds strange but is easily understandable once we remember that "atonement" (kofer) is about covering an offense from view: Phinehas attracted all of God's attention. God was so delighted to see someone with a burden for him that he focused on Phinehas instead of those Israelites who were sinning. The sinning Israelites were thus, in a sense, hidden from God's view and so the plague stopped.
Too few of Yeshua's followers have a burden for Adonai. Instead we tend to develop burdens for people. We focus on ways we have helped people in the past, or ways to comfort people whose suffering we understand. We need to have a burden for Adonai, so that we, like Phinehas, can bring mercy and healing to where we act in God's name.
(Tangentially, besides a "covenant of priesthood" God also made a "covenant of peace" with Phinehas. Commentators explain this other covenant as protection from revenge by the tribe of Shimon. Significant to both covenants was the lineage of Phinehas. In Exodus 6:25 we are told his father, Eleazar, married a daughter of a man named Putiel. Strangely, Putiel is not given a lineage, and his name is related to the word pitem, "fattener", used for people who fatten calves for idolatry. Moreover, the name Putiel means "afflicted of God". The traditional interpretation is that Putiel was an Egyptian who worked as a fattener of calves for the Egyptian priests, and who died during the plagues. Thus Numbers 25 repeats twice that Pinchas was Aharon's grandson because the tribe of Shimon was insulted when Zimri, one of their princes, was killed by someone whom by birth was only half-Israelite!)