Piano
The term "piano" is short for "pianoforte," which means "soft-loud" in Italian. This refers to the piano's ability to produce both soft and loud sounds, which is achieved by the use of hammers striking strings. The piano is a relatively modern instrument, having been developed in the 18th century.
Clavier
The term "clavier" is a general term for any keyboard instrument. This includes pianos, as well as other instruments such as harpsichords and organs. The term "clavier" is often used to refer to older keyboard instruments, such as those that were popular in the Baroque period.
Summary
In general, the term "piano" refers to a specific type of keyboard instrument that is capable of producing both soft and loud sounds, while the term "clavier" is a general term for any keyboard instrument.