1. The act of drawing in the breath.
2. Drawing in a medicated vapor with the breath.
3. A solution of a drug or combination of drugs for administration as a nebulized mist intended to reach the respiratory tree.
4. Drawing in or exposed to an agent (gas, vapor, or particulate) that is contained in inspired or inspirable air.
The definition information for inhalation is provided by Stedman's. You can search our medical dictionary here.