(n.) For disk quotas, a maximum limit on file system resources (blocks) and inodes) that users cannot exceed.
A preset limit (e.g., of functionality) that cannot be exceeded.
A hard limit is a stop on the machine that prevents the machine from moving further in a given direction. Typically these are used to prevent the machine from moving beyond its physical limits. See Soft Limit.
Although limits are things that are not done in a scene in BDSM, play is often used as a way to expand on such limits. A hard limit, on the other hand, is an automatic pre-agreed line never to be crossed, as it would be a breach of acceptability.