Answer:
The piano is certainly an orchestral instrument, but only from the 20th century (before that it would only play with an orchestra as a soloist).
In f.e. Petroushka by Stravinsky it plays an important role. Schostakovich uses piano in many of his symphonies; Bartók, Copland, Falla use it etc.
In contemporary orchestral pieces it is very common to include piano in the orchestration.