Masking, to a "professional", is called "social camouflage", because NTs apparently found adding unnecessary syllables and jargon preferable to adopting a perfectly good term from NDs.
Masking, to me?
Masking is internalizing from an early age that people are hostile to those different from them, and the less obviously different I am, the less I become a target for random aggression. Learning to avoid notice altogether is quite useful. More than one person has noted my ability to be invisible in plain sight.
Masking is suppressing my empathy, learning to turn it off like a light switch, to prevent it being used as a weapon against me. Masking is learning to be cruel when necessary.
Masking is learning to read others, and to read social hierarchies, and cliques. Masking is deciding that if I'm going to be labeled no matter what I do, I may as well dress or act or speak certain ways, and thus choose my own labels.
Masking is analyzing everything that everyone says around me, scanning tone, scanning the words for literal meaning and for known possible subtexts, determining what merits a reply and what sort of reply, and having to do it extremely quickly, which is hindered by issues with speech processing and sound filtering.
Masking is learning to disguise my stims or forego them, so as to avoid unwanted attention. As a child, it was bullying, as an adult it's mockery or suspicion of drug use and mental illness by authorities.
Masking is learning a bunch of meaningless small talk scripts, learning which ones to regurgitate on command, learning to randomize phrasing to seem less "canned", and occasionally looking up popular things to provide conversation fodder. A quick Wikipedia browse is much faster than sitting through a television show.
Masking is suppression of pain from sensory sensitivities. Masking is learning to partially shut down rather than melt down in the face of sensory and cognitive overload, so as to maintain the facade, and preserve enough function to avoid helplessness when in public.
Masking is crawling home exhausted, burnt out, unable to do anything of meaning, because of the effort spent processing speech, analyzing social situations, reading rooms, suppressing pain, suppressing the urge to lash out, to make them all shut up, to stop the noisy things and the insinuations and their damnable staring eyes.
Masking is forcing myself to follow the whims, routines, and rituals of others, contrary to my own interest, contrary to my fundamental nature, every day. When I inevitably fail, masking is hating myself for the days I cannot.
All of that, for me, is or has been masking. Some of the more screwed up behaviors I'm trying to unlearn, and I'm trying to learn to mask less. It's a work in progress.