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Definition of Semi-norm
1. Noun. (analysis) A function denoted ||''v''|| that maps a vector ''v'' to a non-negative value such that ||''cv''|| = |''c''|.||''v''||, where ''c'' is a scalar, and ||''v'' + ''w''|| ≤ ||''v''|| + ||''w''|| (the triangle inequality); the condition that ||''v''|| = 0 implies that ''v'' = 0 is not required, but when it holds, the semi-norm is a norm. ¹
¹ Source: wiktionary.com