The original name was "sideways addition". There are many other names, including Hamming weight. The Wikipedia entry for Hamming weight says the HP-16C developers agree with you:
> Some programmable scientific pocket calculators feature special commands to calculate the number of set bits, e.g. #B on the HP-16C[3][17] and WP 43S,[18][19] #BITS[20][21] or BITSUM[22][23] on HP-16C emulators, and nBITS on the WP 34S.[24][25]
