In simple terms ‘material’ as an adjective means important. In legal practice its meaning is somewhat more nebulous, but it generally considered to mean something that is sufficiently important to potentially change an outcome. Thus it could be a fact that might have deterred a party from entering into a contract or commercial agreement or an item of prior art that would have cause a patent office to reject certain claims of a patent as written, or a court, tribunal or jury to reach a different conclusion on a legal matter or issue. In terms of contractual performance, a material breach is usually something that means that one party is not receiving what they bargained for, where the detriment is sufficient for the party to bring a suit of claim for breach of contract.
By contrast non-material or de minimis refers to things that are unlikely to have changed an outcome or a party’s position.
Whether something is ‘material’ or not can prove a very contentious issue; while most oqbervers can identify facts or breaches of contract that are clearly ‘material’ or clearly ‘not material,’ in practice there may be a lot of close questions as to materiality. For those reason it is common in contracts to identify certain itemised potential breaches as ‘material’ so as to be clear as to what terms are of particular importance to the parties, usually with the rider “not limited to” to prevent the application of the inclusio unius, exclusio alterius principle.