WordNet sense 1 (the interchange of sections between pairing homologous chromosomes during the prophase of meiosis):

crossing over

WordNet sense 2 (a voter who is registered as a member of one political party but who votes in the primary of another party):

crossover voter

WordNet sense 3 (a path (often marked) where something (as a street or railroad) can be crossed to get from one side to the other):

crossing, crosswalk