1. Substance in appreciable amount which, when associated with a trace of a specified substance, will carry the trace with it through a chemical or physical process.

2. Person who is heterozygous, that is carries only one allele, for a recessive genetic character leading to disease, and hence does not, under most circumstances, display the disease phenotype but can pass it on to the next generation.

3. Gas, liquid, or solid substance (often in particulate form) used to absorb, adsorb, dilute or suspend a substance to facilitate its transfer from one medium to another.