You've heard of glazing, and scumbling. Velatura is another art term. It means applying a semi-opaque layer of paint, usually but not exclusively oil paint, over another, and it's not a term I explained in my e-book Oil Paint Basics (still available from the Amazon Kindle store. When I produce a revised edition, I'll add it.)
Velatura is the application of a layer of paint that part obscures the paint over which it's applied: it doesn't completely obscure it, and is most often employed in portraiture. It can be very effective, and will often be facilitated with Zinc White, although it can be applied just as well with thinned Titanium White.
It isn't actually necessary that you know these terms: many of us have long since applied the techniques without knowing what words might describe them: and anyway, what makes the term 'en plein air' any better than 'out of doors' painting? But it may help to know what it means when reading art books without having recourse to the dictionary. If you've ever painted a portrait, you've probably applied the technique without knowing the words to describe it: now and then, though, the words can alert you to a technique you wouldn't otherwise have thought of trying.
Pedantry can be good ..................