Recently, I've found myself talking about the use of pinching to zoom out on mobile devices, and I've been struggling with an opposite for the word 'pinch'. In lieu of a better word, I've been saying 'unpinch'. I'm wondering if there's a more appropriate word to use to describe the motion of spreading the fingers from a pinch position.
In their own iOS Developer Library, Apple calls it pinching in and pinching out:
Since Apple is the company that brought pinching to to such widescale usage as it has been in recent years (and, as far as I am aware, though I have no sources to back this up, also the company that coined the extended meaning of pinch for this thitherto unnamed gesture), I’d say their terminology is more or less as definitive as you’re going to get in this particular context.
I have occasionally heard the term stretch when it comes to touch screen manipulation ("Pinch 2 or more fingers together to zoom out. Stretch them apart to zoom in.") which may be suitable so long as the user interprets it as zooming in rather than resizing an image. Just keep in mind that stretch might imply resizing one dimension and not the other.
Side note: Certain articles define pinching as "when two touch points are detected on the touch screen" regardless of the direction the fingers are moving. So I guess there's no official term.
The answer @Janus Bahs Jacquet posted is an interesting piece of etymology and most likely the closest we'll get to a temporary standard from the current state of the industry.
As neat as the tidbit of info is, I think both pinch-in and pinch-out are basically yet-another-example of dogmatic Apple-lingo, by no fault of Janus.
Since you are asking about appropriateness, I would suggest that you have already said what may be the most appropriate word for the inverse of the natural pinching action (as opposed to terminology).
The word: spreading.
As further proof, notice how easily pinch and spread are paired with two other antonyms, together and apart while being just as friendly to the in/out pair.
Saying, "pinch-in with your fingers to zoom-in", "zoom-out by pinching out" or generally pinching in any direction seems a bit odd verbally.
I think that's because pinching needs to correlate things closing in on each other. Speaking to the younger kids in my family we'll say "don't let that door pinch your finger." More abstract references like "I'm in a pinch" may refer equally to a "tight situation" where the person may be "squeezed for time/money/other".