The Spanish demonstrative pronoun 'aquello' (meaning 'that') comes from the Latin phrase 'eccum ille'. This Latin phrase combined two words: 'eccum' meaning 'behold, there' and 'ille' meaning 'that'. Over time, this combination 'eccum ille' evolved phonetically to become the modern Spanish 'aquello', while maintaining its demonstrative meaning of 'that'.

This evolution shows how Latin's more complex system of demonstratives, which often combined particles for emphasis, simplified into Spanish's three-way demonstrative system (este/ese/aquel and their variants).

'Aquello' belongs to the same demonstrative family as the more commonly used Spanish words 'aquel' (that) and 'aquella' (that). These words are used frequently in basic Spanish and share the same Latin origin. While 'aquel' is used with masculine nouns and 'aquella' with feminine nouns, 'aquello' is used as a neutral demonstrative when referring to undefined or abstract things.

While English 'that' is not etymologically related to 'aquello', there is an interesting connection to some English words borrowed from Latin. The Latin 'ille' that helped form 'aquello' is also the source of pronouns in Romance languages and appears in English words like 'illegal' (meaning 'not legal', where 'il-' comes from 'in-' + 'ille') and can be seen in borrowed phrases like 'le roi' (the king) in French loanwords, where 'le' derives from 'ille'.