The Spanish word 'nulidad' (meaning 'nullity' or 'invalidity') comes from the Latin word 'nullitas' meaning 'nothingness'. This Latin word was formed by combining 'nullus' (meaning 'none') with the suffix '-itas' which was used to form abstract nouns. Going even further back, 'nullus' itself was created from two parts: the negative prefix 'ne-' meaning 'not' and 'ullus' meaning 'any', literally forming 'not any' which came to mean 'none'.

This etymology shows how abstract concepts like 'nothingness' or 'invalidity' were built up from simpler negative expressions in Latin before entering Spanish.

Some related Spanish words include 'nulo' (null, void), which comes directly from Latin 'nullus', and 'anular' (to nullify, to cancel), which shares the same root. These words form a family of terms relating to the concept of invalidity or non-existence in Spanish.

English speakers can easily connect 'nulidad' to several similar English words that share the same Latin origin: 'null', 'nullity', 'nullify', and 'annul'. All these words maintain the core meaning of 'nothing' or 'making something invalid' that was present in the original Latin. The academic or legal term 'nullity' is particularly close in both form and meaning to Spanish 'nulidad'.