I just noticed this word for the first time today; it means “the act of obtaining.” A sign of its rarity is that “obtainment” is a much more common synonym — and “obtainment” isn’t exactly super popular. Google Ngrams tells us that “the obtaining of” is much more common than either “the obtainment of” or “the obtention of” (especially British English, but also in American English). Westlaw likewise reports that “obtention” has only been used 7 times since 1/1/2010.