From what I've seen "covenant" is the most common term used for this kind of thing. For example, here's a page I put together once to help me understand the differences between the various patent promises that might affect ODF and OOXML, although half the links to the originals no longer work. It shows two Microsoft documents, a Sun documents, and the terms IBM uses for many standards side by side, with the text colored so that sections in the difference ones that correspond in function are colored the same. It makes it easy to look at one and see how the others word their similar section:
In essence, a "covenant" not to sue is a fancy way of saying "promise" not to sue.
As far as the difference between a covenant/promise and a license goes, the courts have been unclear in their guidance. Covenants/promises may not survive a sale of the patent--although the cases are mixed. Covenants/promises may not provide the person they are given to the right to sell things that use the patent to third parties (or rather, it may not apply to the third parties and so the patent owner might be able to sue them). The case law here is murky, too, with the latest tending to favor that the covenant/promise does apply downstream.
I've seen cites to at least one appeals court case where the court said all patent licenses are really just promises not to sue. That's because a patent does not give the patent owner permission to practice the patent. It just gives him the power to exclude others from doing so, and so he can't really license you the right to actually make something that is covered by that patent. All he can do is agree not to sue you if you do so.
Most of the cases, though, apply to covenants/promises to a specific party. E.g., company X and company Y are disagreeing over X's patent, and as part of some settlement X grants Y a promise not to sue.
Different considerations might apply when the covenant/promise/license is not to anyone specific but is rather a non-exclusive covenant/promise/license to everyone. When viewed as a license, this kind of license is something called a "bare license". Bare licenses are not good for the recipient, as they can be revoked at any time. What you really want is for the covenant/promise/grant to be viewed as a contract. Fortunately, there is a mechanism in contract law to allow this, called promissory estoppel. The basic idea is that if I make a promise to you (where you can be the whole world), reasonably knowing that you are going to rely on this promise, and you do rely on this promise and act accordingly, then the law will enforce the promise.
http://www.tzs.net/odfooxmlpatent.html
In essence, a "covenant" not to sue is a fancy way of saying "promise" not to sue.
As far as the difference between a covenant/promise and a license goes, the courts have been unclear in their guidance. Covenants/promises may not survive a sale of the patent--although the cases are mixed. Covenants/promises may not provide the person they are given to the right to sell things that use the patent to third parties (or rather, it may not apply to the third parties and so the patent owner might be able to sue them). The case law here is murky, too, with the latest tending to favor that the covenant/promise does apply downstream.
I've seen cites to at least one appeals court case where the court said all patent licenses are really just promises not to sue. That's because a patent does not give the patent owner permission to practice the patent. It just gives him the power to exclude others from doing so, and so he can't really license you the right to actually make something that is covered by that patent. All he can do is agree not to sue you if you do so.
Most of the cases, though, apply to covenants/promises to a specific party. E.g., company X and company Y are disagreeing over X's patent, and as part of some settlement X grants Y a promise not to sue.
Different considerations might apply when the covenant/promise/license is not to anyone specific but is rather a non-exclusive covenant/promise/license to everyone. When viewed as a license, this kind of license is something called a "bare license". Bare licenses are not good for the recipient, as they can be revoked at any time. What you really want is for the covenant/promise/grant to be viewed as a contract. Fortunately, there is a mechanism in contract law to allow this, called promissory estoppel. The basic idea is that if I make a promise to you (where you can be the whole world), reasonably knowing that you are going to rely on this promise, and you do rely on this promise and act accordingly, then the law will enforce the promise.