As with many of relationship topics, a certain amount of attention to what we mean by particular words is helpful and often necessary when discussing jealousy.
It is important to distinguish jealousy from envy. To oversimplify, jealousy is angry, envy is wistful. Jealousy is destructive, envy rarely is.
Insecurity, as used below, just means a lack of security. It does not imply that the insecure person is bad, or good. It does not imply that the insecurity is warranted or unwarranted. It simply means that the person is not secure, either in themselves, their relationship, or what have you.
Jealousy is Insecurity
This is an important point. Feelings of jealousy always appear to stem from one's sense that something about their life is not secure, e.g., is uncertain or in danger.
Several readers have objected to this particular point because they believe that insecurity is necessarily pejorative. That's not what I mean by the word. (I'd love a better word, but I don't have one.) It could be that this lack of security is very well founded--that the partner is about to run off with 'the other woman'. Is it okay to be bothered by that? Of course it is.
In some cases, the insecurity is not founded on realistic dangers to the relationship. If that is really the case, then you (the jealous partner) may wish to consider where your insecurities are coming from. Solving those sorts of insecurities isn't easy, but until you do you'll continue to face those feelings.
Since we tend to become more secure in relationships as they become more stable with time, you may find that time is your ally in dealing with jealousy.