The word means "below the level of consciousness" and was applied in the 1950s to images in advertising that were intended to influence the viewer without that viewer realizing what he had seen.
This might be an erotic image or other suggestive content cleverly inserted into a photograph or a single frame of film in a TV commercial. The single frame might be the words "BUY THIS!" Whether it was actually effective in marketing remains questionable.
In animation, I am coining the term subliminal anticipation to cover a technique described by Richard Williams in his book The Animator's Survival Kit. Williams doesn't use the word subliminal, instead referring to Invisible Anticipations on page 283 of his book (the first edition.)
Like most of the tips and tricks Williams describes, this is a subtle trick learned from Hollywood animators from the Warners and Disney studios to whom he "apprenticed" himself in the 1970s and 80s.
Unlike the obvious anticipations with which most of us are familiar, such as the windup of a baseball pitcher before the pitch--easily the most drawn-out anticipation example that I can think of--, the subliminal anticipation happens so fast that it isn't actually seen, but only felt. The images of the anticipation are shot on ones rather than twos (in terms of 24fps film speed), faster than the eye can register, and yet as Dick Williams puts it, they add a "snap" to an action that can be most effective.
I used this technique before an accented syllable in the dialog animation being analyzed in posts 132, 133 and 134 of this blog. As he speaks, the Old Man is lowering his head. One an accent, his head suddenly jerks upward and begins descending again on a new path. Before the accent is where I inserted two frames of subliminal anticipation.
First, let's look at a simplified example of the anticipation and accent using only a simple ellipse as the object.
Now, here is the same effect employed in dialog animation of the Old Man from my film in progress, Carry On. I used this subliminal anticipation technique on both the syllable accents (IN-ternet and FA-il.)