How to correctly name a multi-select element in forms?

How to correctly name this in English (in an official documentation for an IT system or in an instruction for filling up some form):

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Should I name this:

  • a checkbox,
  • a check box,
  • a tickbox,
  • a tick box,
  • other name.

According to Wikipedia, all four options are good. But checkbox (as one or two words) is used much more often, according to Google Ngram, and (being a software engineer myself) the term I hear/see the most. As an HTML input element, it's also called a checkbox.

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