x1 is a binary operator which is associative in space/under conditions/on (or endowing) set x2.
Denote x1 by "♤"; for any elements x,y,z in the set of concern, if '♤' is associative, then (x♤y)♤z = x♤(y♤z), where equality is defined in the space and parenthetic grouping denotes higher prioritization/earlier application. An operator being associative usually results, notationally, in the dropping of explicit parentheses. "Associative property"/"associativity" = "ka(m)( )socni". Notice that a space is understood to support association (or 'be associative') under a given operator x1 in the sense that its elements and equality relation are such that all ordered pairs can be exchanged in application under the operator while maintaining equality; this is similar to a space being endowed with identity element(s) under a given operator and also an operator have identity in a space - in English, associativity is often understood to be a property of a function and the existence of an identity to be a property of a space, but they are wed in Lojban, as they should be, for neither makes sense without the other and the property is a relationship between the operator and the space. See also: cajni, sezni, dukni, facni.