The concentration of a compound which produces an odour which is detectable by a human being (nose). For certain compounds this threshold is very low (e.g. 1 part in 10E9 for certain sulfides).
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2204 (https://doi.org/10.1351/pac199062112167)