What type of literary technique is the phrase 'star-crossed lovers' in 'Romeo and Juliet'?

From "Transactions - Volume 20" - Page 60 "Homoeopathic Medical Society of the State of New York" • 1885 (books.google.co.uk › books)

An examination of astrology, apart from astronomy, as the art of judging of the influences of the stars, and of foretelling future events by their position and aspects, would be a postmortem. Yet the decease of the art occurred not long ago, for as late as the middle of the seventeenth century belief in astrology was almost universal.

Other readily available sources confirm this.

What type of literary technique is the phrase 'star-crossed lovers' in 'Romeo and Juliet'?

"star-crossed" is a straightforward adjectival phrase, and a statement of, what was then, accepted. It would be seen as no different from "a pair of young lovers"

There is no "metaphor".

'Star-crossed lovers' are two people who, although in love, have natal charts which are 'crossed'. 'Star-crossed' is, effectively, a technical term in astrology indicating that there are aspects of the charts of the two lovers which, when compared, show that the forces of fate will tear them apart or, at least, prevent them from being happy together.

In detail there will be aspects of the planets (wandering stars) and other parts of the charts which have powerful ninety degree angles between them (a ninety degree angle is a 'square aspect' in astrology and is usually considered to be an indicator of negative forces operating on the subject of the chart. Four planets with square aspects to each other can form a "Grand Cross" which is considered to be a very negative element of a chart). A true believer in astrology would, of course, say that there are also aspects of the combined charts which led them to fall in love in the first place.

You don't have to believe in astrology, and certainly not in the fixed fate interpretation of it, to consider that Renaissance people would have taken the term literally. From our perspective it is metaphorical but from Shakespeare's perspective it was not.

If we write "star-crossed lovers" in most contexts in the 21st century then the phrase is metaphorical in the same way that calling someone 'phlegmatic' is metaphorical, but in the late 16th and early 17th centuries when Shakespeare was writing all educated people accepted the validity of astrology and the existence of the 'four humours of the body' so 'star-crossed' and 'phlegmatic' were literal uses of technical terms for them.

Epithet

In English, I would say that 'star-crossed lovers' is an 'epithet', which is an alternate, usually descriptive, sometimes complimentary or it can be disparaging, name, ascribed to somebody. Or in this case, to somebodies - to a couple.

An epithet is, according to Cambridge Dictionary online: 'An adjective added to a person's name or a phrase used instead of it, usually to criticize or praise them: The singer's 600-pound frame earned him the epithet of "Man Mountain" in the press. https://dictionary.cambridge.org/dictionary/english/epithet

Other examples might be:

Fagin, the 'light-fingered' character from Oliver Twist, slid his hand into the rich man's pocket. - Meaning, he is a thief

Elidor, 'the light bringer', landed on the grass, his nostrils flaring, neighing, tossing his horn at the moon Meaning, the unicorn brings light

The epithets are in quotes.