accidental(happening by chance) adjective [UK: ˌæk.sɪ.ˈden.tl̩] [US: ˌæk.sə.ˈden.tl̩]That discovery was quite accidental. = Ese descubrimiento fue totalmente casual.
casual(happening by chance) adjective [UK: ˈkæ.ʒʊəl] [US: ˈkæ.ʒə.wəl]John casually shrugged. = John casualmente se encogió de hombros.
nonchalant(casually calm and relaxed) adjective [UK: ˈnɒn.ʃə.lənt] [US: ˌnɑːn.ʃə.ˈlɑːnt]
casualidad noun {f}
chance [chances](random occurrence) noun [UK: tʃɑːns] [US: ˈtʃæns]I met him by chance. = Me lo topé por casualidad.
coincidence [coincidences](of objects, property of being coincident) noun [UK: kəʊ.ˈɪn.sɪ.dəns] [US: koʊ.ˈɪn.sɪ.dəns]It was just a coincidence. = Fue pura casualidad.