Why are Romeo and Juliet called "star-crossed lovers"?
A pair of star=crossed lovers take their life,
Whose misadventured piteous overthrows
Do with their death bury their parents" strife. (1.1.6-8)
In the Prologue of Romeo and Juliet that is spoken by the Chorus, who introduce the play to the Elizabethan audience, the term "star-crossed" would be one quite familiar to these Elizabethans. For, the stars are part of the Chain of Being. When one part of this chain is upset, as in the stars, then there is disorder and chaos. So, when the stars are taken out of their order, things go awry and fate changes the order of things.
Therefore, when Romeo and Juliet have their stars taken out of order and "crossed," they become fated lovers, lovers to whom misfortune will come. Thus, their lives contain a destiny that will prove tragic for them. In modern times, one would say that they have bad luck, such as when they meet and their families are in a feud against one another; or, when Romeo happens upon Mercutio and Tybalt fighting and he inadvertently kills Tybalt; or, when John cannot get the Friar's message to Romeo because Mantua is quarantined; or, when Friar Laurence runs out of the catacombs and Juliet is left alone to her fate.
The term "star-crossed" refers to the idea that those involved will be impacted in a negative way. The term generally refers to a couple that due to some unfortunate circumstance, some uncontrollable, pre-determined fate are destined for failure.
In Romeo and Juliet, the title characters are impacted by many factors that are beyond their control, primarily caused by family feuding. The "bad-luck" that plagues the relationship from the beginning foretells a sad tale of youth, conflict and an attempt to be together despite the painful realities of their situation. In the end, the couple falls to an eternal rest; while those who love them mourn their death.
Romeo and Juliet is not the earliest example of star-crossed lovers, but it is among the most famous.
The term means that they are doomed by fate. The whole idea of astrology (which was much more believed in back then) is that the stars have something to do with what happens to us. Therefore, if someone is "star crossed" they have been dealt a bad hand by fate.
This has happened to Romeo and Juliet because they have had the bad luck to be born to families that hate one another. This means that, when they fall in love, they cannot just get married like other people might.
There are numerous other instances of bad luck in the rest of the play, but this is the main one.
So -- bad luck = star crossed.
what they said ^
You know from the prologue that introduces the couple as "star-crossed" that the couple is doomed, hence the title "The TRAGEDY of Romeo and Juliet." By the end of the play, you see how the couple (regardless of how grand their intentions) is at the mercy of chance/fate/destiny/bad luck.