Las definitions are a key element of any reasoned knowledge. However, there are frequent confusion about the fate they actually play in language. Against this background, Wirtgenstein’s philosophical perspective helps us to overcome the perplexity that arises when we do not find precise and unambiguous references for certain definitions, and suggests that those definitions rather reflect the rules of use of concepts in specific contexts. If we take this into account in the area of law, for example, we will avoid attempting that the definitions specific to legal codes establish the ‘true nature’ of the concepts dealt with therein.