Definition of "uncheck"

uncheck

/ʌnˈt͡ʃɛk/

verb
third-person singular simple present unchecks, present participle unchecking, simple past and past participle unchecked
1. (graphical user interface, US, Canada) To remove a checkmark.
  Quotations
  If you uncheck either of these check boxes, [Microsoft] Expression Web 2 will create all new Web pages with uppercase tag names and/or attributes. Current Web standards require that HTML tag names and attributes be lowercase. Therefore, pages created with either of these boxes unchecked will not be compatible with existing standards.
  2008 March, Jim Cheshire, “Using Web Page Views”, in Using Microsoft Expression Web 2, special edition, Indianapolis, Ind.: Que Publishing, Pearson Education, page 138
2. (travel, aviation) To remove a previously checked-in passenger from the check-in system, allowing changes to the ticket.