A character type in which numerically increasing changes are allowed and counted as for ordered characters, while decreasing changes are not allowed (i.e. counted as an infinite number of steps); thus, multiple reverse changes (= losses) are not allowed. By definition, an irreversible character is polarised in advance, making the use of an outgroup redundant. This character-type is very rarely used, as the assumption of irreversibility is very difficult to justify for any type of data, morphological or molecular. It was proposed by E.O. Wilson (1965, Systematic Zoology 14:214-220), with examples of its application (cf. ordered, unordered, Dollo).