late 14c., conservacioun, "preservation of health and soundness, maintenance in good condition, act of guarding or keeping with care," from Latin conservationem (nominative conservatio) "a keeping, preserving, conserving," noun of action from past-participle stem of conservare "to keep, preserve, keep intact, guard," from assimilated form of com-, here perhaps an intensive prefix (see com-), + servare "keep watch, maintain" (from PIE root *ser- (1) "to protect").
Meaning "preservation of existing conditions" in any sense is from mid-15c. The word has been used since late 15c. in reference to English municipal authorities who had charge of rivers, sewers, forests, fisheries, etc. Specifically with reference to preservation of nature and wild places by 1909.
The phrase conservation of energy apparently was coined in French by Leibnitz in 1692; it is attested in English publications from early 18c. as conservatio virum vivarum or partially nativized versions of it. The exact phrase is attested from 1853.