What’s natural language?

I am often asked this question by customers.
Natural language is the everyday language (English, Italian, German, etc.) used to communicate at all levels, which in computational linguistics is opposed to “formal language”, created expressly for a specific purpose. While natural language changes and evolves continuously thanks to neologisms, idioms, loanwords, slang, etc., formal language is closed and without exceptions: no semantic ambiguity, no omography, no homonimy, limited expressiveness.