exposure
in medicinal chemistry
  1. Concentration, amount or intensity of a particular physical or chemical agent or environmental agent that reaches the target population, organism, organ, tissue, or cell, usually expressed in numerical terms of concentration, duration, and frequency (for chemical agents and micro-organisms) or intensity (for physical agents).
  2. Process by which a substance becomes available for absorption by the target population, organism, organ, tissue or cell, by any route.
Source:
PAC, 2004, 76, 1033 (Glossary of terms used in toxicokinetics (IUPAC Recommendations 2003)) on page 1051
N.B. This differs from the definition of exposure in radiation chemistry.