A shadowban (also called stealth banning and ghost banning)bans a user from a subreddit in such a way that the user is unaware they have been banned—submissions are only visible to the user who submitted them; other users never see them. Topics (brand names, phrases, etc.) and domains (host of content being submitted) can be shadow-banned too. [1]

A user, topic, or domain can be determined as shadowbanned by logging our or using incognito to look in the subreddit's /new section; if the submission cannot be seen, it's likely the result of a shadowban.

Contexts

[1] Shadowbanned, reddit.com