In set theory, a projection is one of two closely related types of functions or operations, namely: A set-theoretic operation typified by the j {\displaystyle j} th projection map, written p r o j j, {\displaystyle \mathrm {proj} _{j},} that takes an element x → = {\displaystyle {\vec {x}}=} of the Cartesian product {\displaystyle } to the value p r o j j = x j. Wikipedia