a prefix with the meaning "round about, around," found in Latin loanwords, esp. derivatives of verbs that had the general senses "to encompass or surround" ( circumference; circumjacent; circumstance ) or "to go around" by the means or in the manner specified by the verb ( circumnavigate; circumscribe ); on this basis forming adjectives in English with the meaning "surrounding" that named by the stem ( circumocular; circumpolar ).
[ circum around (acc. of circus; see CIRCUS, CIRCLE), orig. as an adv. fixed in relation to the v., later construed as a prefix ]