De minimis is a Latin expression meaning about minimal things, which is mostly used as part of de minimis non curat praetor, in the sense that law is not interested in trivial matters.
De minimis, in a more formal legal sense, means something which is beneath judicial notice.
For example, in European Union Competition law companies which indulge in anti-competitive practices, but which have a market share of under 5% often escape censure by the European Commission because such activities are regarded as de minimis.
In risk assessment "de minimis" refers to a level of risk which is too small to be concerned with -- some refer to this as a "virtually safe" level.
Courts will not uphold copyright on modified public domain material judged to be "de minimis" if the changes are deemed to be minor. For instance, a clear-cut "de minimis" case would be trying to assert copyright after simply correcting spelling mistakes in a public domain text.
See also