1630s, "action of bringing to a center, act of collecting or combining into or about a central point," noun of action from concentrate (v.). Meaning "a mass so collected" is from 1670s; that of "voluntary continuous focusing of mental activity" is from 1825, in phrenology.
Concentration camp is from 1901, originally "compound for noncombatants in a war zone," a controversial idea in the second Boer War (1899-1902). The term emerged with a bad odor.
The concentration camp has now definitely taken its place side by side with the Black Hole of Calcutta as one of those names of horror at which humanity will never cease to shudder. [The Review of Reviews, London, March 1902]
But it also was used 1902 in reference to then-current U.S. policies in the Philippines, and retroactively in reference to Spanish policies in Cuba during the 1896-98 insurrection there. The phrase was used domestically in the U.S. during the Spanish-American war but only in reference to designated rendezvous points for troops headed overseas. In reference to prisons for dissidents and minorities in Nazi Germany from 1934, in Soviet Russia from 1935.