Strain is also a physical quantity term.

In biology, Strain can be used two ways.

Microbiology/Virology

A strain is a genetic variant or subtype of a virus or bacterium, or an inbred line of a higher organism. For example, a "flu strain" is a certain biological form of the influenza or "flu" virus.

Biology/Botany

A strain is a group of plants with similar (but not identical) appearance and/or properties. Sometimes incorrectly used as a synonym for variety. Strains are similar to series and groups.