What does the term strain mean as it is used in genetic crosses

In biology, the strain is a low-level taxonomic rank used in different contexts:

In microbiology, a strain is a part of a bacterial species different from other bacteria of the same species by a minor but identifiable difference. Strains are often created in the laboratory by mutagenesis existing strains or wild-type examples of bacterial species.

In zoology, a strain corresponds to an individual or group of individuals who are at the origin of a line of descendants, sometimes called the holotype, paratypes, etc. A strain is a population of organisms that descends from a single organism or pure isolate culture. Strains of the same species may differ slightly from each other in many respects.

A strain thus consists of a group of organisms of the same species possessing certain differential traits based on their relationship; either they come from the same region, as the same watershed of a river, or they are the fruit of a particular breeding program (exists as a whole interbreeding without introductions from external sources).

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.