The Numero sign or Number sign is used in many languages to indicate ordinal numeration, especially in names and titles, for example, instead of writing the long
"Number 4 Privet Drive"
one would write the numero sign so:
"№ 4 Privet Drive",
and spoken as if written out in full.
The numero symbol combines the upper-case Latin letter "N" with a superscript lower-case letter "o", sometimes underlined, resembling the masculine ordinal indicator.
According to the OED, the term is from the Latin numero, which is the ablative form of the word numerus (NVMERVS in inscriptions, meaning "number", with the ablative meaning "to/by/with the number").
Similar forms exist as the word for "number" in Latin-derived languages: numero in Italian, numéro in French, and número in Spanish and Portuguese.