ukhlusiya

What is the origin of the word אֻכְלוּסִיָּה uchlusiya - "population"?

This is a word that entered into Hebrew in the Talmudic period (for example, Yevamot 36b) with the sense of "multitudes", borrowed from the Greek ochlos, meaning "crowd."

Klein says that the Greek word is "perhaps related to ochein (= to carry), and cognate with Latin vehere (= to carry, convey)." 

In the entry for the the related word ochlocracy - "government by the rabble", the Online Etymology Dictionary adds the following:

"government by the rabble," 1580s, from French ochlocratie (1560s), from Greek okhlokratia (Polybius) "mob rule," the lowest grade of democracy, from kratos "rule, power, strength" (see -cracy) + okhlos "(orderless) crowd, multitude, throng; disturbance, annoyance," which is probably literally "moving mass," from PIE *wogh-lo-, suffixed form of root *wegh- "to go, move."  "Several possibilities exist for the semantic development: e.g. an agent noun *'driving, carrying, moving', or an instrument noun *'driver, carrier, mover'. ... An original meaning 'drive' could easily develop into both 'stirred mass, mob' and 'spiritual excitement, unrest'" [Beekes]. For sense development, compare mob (n.). Related: Ochlocrat, ochlocratic; ochlocratical. Greek also had okhlagogos "mob-leader, ochlagogue."

While the Greek word and its English descendants have a negative connotation, implying orderless rabble, the Hebrew uchlusia doesn't have those associations. (Perhaps because the Hebrew word המון already connects means "crowd" and has the sense of a noisy group.) However, beyond the meaning "population" (as in the total number of people in an area), ukhlusiya can also refer to the specific inhabitants of that area, in the collective sense.