The term 'nocturne' as a designation for a music piece was first used in the 1700s, when it referred to a multi-movement piece for an instrumental ensemble, meant to be played at night (usually at events or parties) and then cast aside. Before that the term referred to the nocturnal religious services.
The solo-instrument romantic nocturne as we know it was conceived during the 19th century. The first pieces titled as 'nocturne' were written by John Field, who defined the style of writing singable melodies over simple arpeggio accompaniments. The most famous exponent of the form, however, was Frédéric Chopin, who wrote 21 of them, redefining the nocturne as conceived by Field by taking it to a new level of harmonic complexity and emotional depth.