(1) An instrument for determining pressure or tension, especially an instrument for determining ocular tension. (2) A vessel for equilibrating a liquid (e.g., blood) with a gas, usually at a controlled temperature; originally so named because it was used with a low gas:blood ratio to allow the gas to approach blood oxygen tension and thus serve as a measure of it; now commonly used with a high gas:blood ratio to adjust the blood to the oxygen pressure of the gas. SYN: aerotonometer (2). [tono- + G. metron, measure]