WordReference Random House Learner's Dictionary of American English © 2014
they'll /ðeɪl/USA pronunciation 
contraction. a shortened form of they will:They'll probably be late because of the snow.

Collins Concise English Dictionary © HarperCollins Publishers::

they'll /ðeɪl/ contraction of
  1. they will or they shall

'they'll' also found in these entries:
In the English description:

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