Censorship does not require an act of Congress. Private entities are perfectly capable of censorship. How else could the term "self-censorship" have any semantic value?
Self-censorship only has value when people fear retribution by government.
"I won't use the word retard in my project hosted on github because github's TOS prevents it" is not self-censorship in any meaningful sense of the word. (Just as "Github doesn't let me spam all these users" is not Github censoring people.) Nothing is stopping you from hosting the repo in a wide number of other places.
"I won't insult my employer in this or that nation, because I risk a jail term" is self censorship.
This is nonsense. Self-censorship occurs whenever somebody censors himself. In polite conversation we might call it discretion (or valor). In a fiduciary or confidential relationship, we might call it adherence to the duty of confidentiality. In a contractual relationship, we would call it nondisclosure. In employment, on one hand we might call it professionalism—or self censorship on the other—depending on the context.
Regardless, the term "self-censorship" is not germane to or inherent in only citizen-government interactions.
People self-censor constantly on social media. You're not worried about the government taking a tweet out of context and driving your reputation into the ground, the mob of internet idiots has that covered. Fear of internet mobs causes people to censor themselves, and a definition of "censorship" that doesn't cover uses like that is not a particularly useful one.
Self-censorship as a term has value whenever people fear losing something that is important to them. With governments it might be their their career chances, personal freedom or even their life. With companies it might be your online identity, your data, your social contacts. Imagine being removed from Facebook/Google/Twitter/[your social network of choice] for writing the wrong post/email. While it is certainly orders of magnitude less threatening than being tortured by the secret police, it is still a deterrent.
