Technical term in chemistry and engineering for a water purification method that uses a semipermeable membrane to remove ions and molecules.
Reverse osmosis is widely used for seawater desalination.
逆浸透法 is the full Japanese technical term for reverse osmosis, while ＲＯ is the common English-based abbreviation used in industry.
Compound of 逆 (reverse) + 浸透 (osmosis) + 法 (method). A calque from English 'reverse osmosis'.