WordNet sense 1 (the state of being joined together):

conjugation, junction, colligation

WordNet sense 2 ((astronomy) apparent meeting or passing of two or more celestial bodies in the same degree of the zodiac):

alignment

WordNet sense 3 (an uninflected function word that serves to conjoin words or phrases or clauses or sentences):

conjunctive, connective, continuative

WordNet sense 4 (the temporal property of two things happening at the same time):

coincidence, concurrence, co-occurrence