Is it ensure, insure, or assure?
(Or "prefix syntax") One of the possible orderings of functions and operands: in prefix notation the function precedes all its operands. For example, what may normally be written as "1+2" becomes "(+ 1 2)". A few languages (e.g., lisp) have strictly prefix syntax, many more employ prefix notation in combination with infix notation.
Compare: postfix notation.