strain is a measure of the deformation resulting from stress
(that is, force per unit area); the displacement of one point with respect to
another, divided by their equilibrium separation in the absence of stress.
fragment generally has some equilibrium geometry (bond
lengths, interbond angles, etc.) When the rest of the molecular structure does not impose special
constraints (e.g., bending bonds to
form a small ring). Deviations from this equilibrium geometry are described as strain,
and increase the energy
of the molecule.
Strain in the mechanical
sense causes strain in the chemical sense.