Source: http://en.wikipedia.org/wiki/Bartolomeo_Cristofori
The piano has had many types of instruments pre-dating it, but as it is today the most commonly known such instrument, in this set of documents its name will be used for the family of similar instruments (eg. clavichord, harpsichord, spinet, etc.).
"Piano" is the modern short name for pianoforte, from Italian piano (which means soft), and forte (which means loud), the name given as notes could be played loudly or softly, unlike other keyed instruments of the time (such as the virginal, spinet, or harpsichord)
A pianoforte of Cristofori dated to 1722 - Museo Nazionale degli Strumenti Musicali in Rome
Source: http://en.wikipedia.org/wiki/Bartolomeo_Cristofori
The piano was invented by Bartolommeo Cristofori (1655 – 1731), perhaps in the year 1700.