The Chief Commissioner's Province of Himachal Pradesh came into being on 15 April 1948 by the integration of the hill states, with four districts — Chamba, Mandi, Mahasu and Sirmaur. The first Chief Commissioner was N. C. Mehta.
Bilaspur was not included — it remained a separate state until 1 July 1954.
Shimla town was not included — it was then in Punjab, and came to HP only in 1966.
The name is credited to Acharya Diwakar Datt Sharma: him (snow) + achal (abode) — the lap of the Himalaya.