I believe when most libraries and stores use the term 'ban', they rely on PEN America's definition:
"any action taken against a book based on its content and as a result of parent or community challenges, administrative decisions, or in response to direct or threatened action by lawmakers or other governmental officials, that leads to a book being either completely removed from availability to students, or where access to a book is restricted or diminished." [1]
[1] https://pen.org/book-bans/book-bans-frequently-asked-questio...