Real Dictionary

strain
If strain is put on an organization or system, it has to do more than it is able to do.
singular noun: If you say that a situation is a strain, you mean that it makes you worried and tense.
uncountable noun: Strain is a state of worry and tension caused by a difficult situation.
verb: To strain something means to make it do more than it is able to do.

strain in American English 1
to draw or stretch tight
intransitive verb: to pull forcibly
noun: any force or pressure tending to alter shape, cause a fracture, etc.
transitive verb: to draw tight or taut, esp. to the utmost tension; stretch to the full

strain in American English 2
a begetting
noun: the body of descendants of a common ancestor, as a family or stock

strain in British English 1
noun: the act or an instance of straining
verb: to draw or be drawn taut; stretch tight

strain in British English 2
noun: the main body of descendants from one ancestor

strain in Mechanical Engineering
noun: Strain is a measure of how much a material changes shape under a force. It is equal to the change in any dimension divided by the original value of that dimension.