Latin maxim meaning that the expression of one thing excludes others.
It is generally used as a canon of construction of statutes or other legal documents, indicating that the express inclusion of one or more things of a particular type necessarily implies an intention to exclude others of that type.
By way of example (highlighted in bold): R (Veolia ES Nottinghamshire Ltd) v Nottinghamshire County Council  EWCA Civ 1214;  PTSR 185, per Rix LJ:
“ The second aspect of the [Audit Commission Act 1998] that I would highlight in these preliminary observations is that section 15(3) (with its related provisions in subsections (3A), (4) and (5)) grants a specific exception to the section 15(1) right of inspection so far as any document ‘contains personal information’. ‘Personal information’ is closely defined and cannot extend to commercial confidential information of the kind in issue in these proceedings. … The argument is made (based on the principle of expressio unius exclusio alterius) that the express exception made in favour of personal information prevents any consideration of a further exception in favour of confidential information.”