The carrying out of what was planned. Execution is where a design meets what is actually there.
Latin exsequi, ex- (out) plus sequi (to follow); the noun for the carrying-out.
Its root is a following all the way to the end, the same root as consequence: nothing is executed by will alone, only by tracking a plan through however reality answers back. Execution is where that following runs out of paper to walk on and has to cross real ground.