Literally an agreement to refrain from asserting a legal right, thus for example a landowner can grant a license to another person to enter their land in a way which would otherwise be trespassing. Similarly, an intellectual property owner has exclusive rights to do certain things, but can license others to do what otherwise would be infringement of those exclusive rights.
A license is sometimes described as a covenant-not-to-sue. However, it can be argued that a license goes further than simply an agreement to refrain from asserting a right, but is in fact affirmative permission to do the acts licensed. See Non-Assert.
A license is usually governed by the choice of law rules applicable to contracts and not to the law of the jurisdiction that conferred the IP rights at issue. It is important to specify the exclusionary rights the license is granted under. Absent such a definition the license will usually be taken to apply to all exclusionary rights held by the licensor.
In European English licenCe is the noun, licenSe the verb; in north-American English licenSe is the more commonly used form, both as a verb and noun, and licenCe usually regarded as an alternate spelling.