相互

each other / mutual

each other / one another / mutually / fret on the neck of a pipa 琵琶[pi2 pa5] (a fret on the soundboard is called a 品[pin3])

appearance / portrait / picture / government minister / (physics) phase / (literary) to appraise (esp. by scrutinizing physical features) / to read sb's fortune (by physiognomy, palmistry etc)

mutual

to interact / interaction / interplay

strong interaction (in particle physics) / strong force

nuclear interaction

mutual assured destruction

mutually compatible

interrelationship

electromagnetic interaction (between particles) / electromagnetic force (physics)

weak interaction (in particle physics) / weak force