An agreement not to assert a patent or patents against the recipient for a period of time or indefinitely. Many recipients believe that it is the equivalent of a licence, but this is not necessarily the case. Unlike a licence, a non-assert may arguably leave the recipient liable for damages during the period it was in force, in the event that it is terminated, although the recipient may be able to raise equitable defenses such as laches or estoppel.
The other key difference between a non-assert and a licence is that while a licence can be argued to be affirmative permission to do something, a non-assert is a passive agreement not to take prohibitive action, at least for a period of time. Fundamentally, with a license, the recipient knows what it has received; with a non-assert, what is received is unclear. As with licences, a non-assert is usually governed by the choice of law rules applicable to contracts and not the law of the jurisdiction that conferred the IP rights at issue (except with the possible exception of acquiescence-based defenses).