1. the simultaneous occurrence of things or events.

2. the rhythmic coordination of speech and movement that occurs nonconsciously both in and between individuals during communication: In self-synchrony, the individual’s bodily movements (e.g., hand gestures, head movements) tend to be synchronized with his or her own speech; in interactional synchrony, the movements of the listener correspond with the speech and movements of the speaker. The latter term also denotes an interaction between a parent and infant that is characterized by attunement even to small shifts in the responses or signals between them. Parent–infant synchrony in the first months of life has been shown to predict secure attachment in the infant and to promote the child’s later emotional and social development. [originally studied in the 1970s by U.S. psychologist William S. Condon and U.S. psychiatrist Louis W. Sander (1918–2012)]

3. in dance therapy, the act of moving together in harmony, which tends to bring people into greater emotional closeness. —synchronous adj.