My best answer comes from an answer that was given to this same question in 2006:
Adult Myers is introduced in the screenplay as a shape jumping on the
back of Marion's car. Therefore, for script consistency, the term was
used for the rest of the draft and eventually the series.
Also, there is further explanation and links to the script here.
The Shape is what John Carpenter referred to Michael Myers as in the
script. Throughout the movie you only see the outline of Michael
lurking in the shadows, non descript and very much a "shape".
In the actual script, the first usage of "the shape" is used to
describe the mask Myers wears...
"Darkness, with a small shape in the center of the screen. As MAIN
TITLES CONTINUE OVER, CAMERA SLOWLY MOVES IN on the shape.
We get closer and closer until we see that the shape is a Halloween
mask. It is a large, full-head platex rubber mask, not a monster or
ghoul, but the pale, neutral features of a man weirdly distorted by
the rubber. ."
Subsequently, the mask (originally designed by Don Post (later
becomming Don Post Studios)) was referred to as "The Shape"
EDIT: It should be noted, much to my dismay, that while watching the movie with the director's commentary it was revealed that the mask is actually a halloween mask of my boyhood idol, Captain James T. Kirk of the Starship Enterprise, painted white. Ugh. My image of him is forever ruined...