Original: Even after the opening, legally the NHM remained a department of the British Museum with the formal name British Museum (Natural History), usually abbreviated in the scientific literature as B.M. (N.H.) or BMNH.
Simplified: The NHM was at first a department of the famous British Museum. When the present NHM building was opened in 1881, it was then called the British Museum (Natural History) or BMNH.