Hard limit
Something a person will not do under any circumstances and that is not open to negotiation, as opposed to a soft limit, which they are hesitant about but may approach gradually.
A hard limit is an activity a person refuses outright, and it is not open to negotiation. Wikipedia’s Glossary of BDSM contrasts it with a soft limit, an activity someone feels unsure or anxious about, which may sometimes be tried cautiously or in a gentle form. Limits are agreed during negotiation. The Portland Leather Alliance’s newcomer guide advises partners to settle limits, health issues and risks before a scene, notes that some people prefer to discuss limits rather than an exact plan, and reminds players that no negotiation covers everything, which is why a safeword is still needed. For rope, Anatomie Studio adds that each person decides how much restricted circulation they will tolerate, as part of their own risk tolerance, and that this choice deserves respect.