In music, rhythms or beats that are unexpected or sound "off" in an interesting way are syncopated. Typically, a syncopated beat puts the stress where it wouldn't usually be.
When your ear is expecting a weak beat and instead hears a strong or stressed one, it's syncopated. This adjective can also describe rhythms that are uneven or that disturb the flow of music and take it in a different direction. As a verb, syncopate means to make music with this kind of rhythm and also to "shorten words by omitting syllables," from the Greek root synkope, "contraction of a word."