I find this word in "Personal life and off-screen work" section of Actress "Mélanie Laurent" wikipedia article:
In a 2009 interview, she explained that she does not have ambitions to be a crossover actress.
What does it mean?
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It's not completely clear but from the context of describing her as working primarily in French Cinema it seems that the implication is that she has no intention of basing herself in the US.
The subtext to this is that working in 'Hollywood' is, in some ways, seen as the ultimate success for an actor, certainly in terms of money and global exposure.
Equally french language cinema has a perception of being an bit niche as English tends to be the default language for global market films where the big money is.
Crossover is also sometime used for actors who are notable as both theatre and mainstream cinema performances. People like Judy Dench and Ian McKellen being obvious examples.