contraction
noun
con·trac·tion
| \ kən-ˈtrak-shən
\
\
Definition of contraction
1a
: the action or process of contracting
The hot metal undergoes contraction as it cools.
: the state of being contracted
b
: the shortening and thickening of a functioning muscle or muscle fiber
c
: a reduction in business activity or growth
d
: the act of acquiring or incurring (something, such as a debt) or catching (something, such as an infection)
contraction of pneumonia
2
: a shortening of a word, syllable, or word group by omission of a sound or letter
also
: a form produced by such shortening
"They'll" is a contraction for "they will."