The Free On-line Dictionary of Computing (30 December 2018):

single static assignment

    (Also known as SSA form) A special form of code
   where each variable has only one single definition in the
   program code. "Static" comes from the fact that the definition
   site may be in a loop, thus dynamically executed several
   times.

   SSA form is used for program optimization or static analysis
   and optimisation.

   (2003-04-12)