What does "Associate Faculty" mean?

This usage of "Associate Faculty" may be fairly unique to the Perimeter Institute. They have a number of faculty with joint appointments at Canadian universities, who typically alternate between semesters with the usual teaching duties at the university and semesters at PI with no responsibilities except for research.

There is a list of such faculty here; you can see that they all have an additional affiliation outside of PI.

The Perimeter Institute may have its own idiosyncratic usage, but generically "Affiliate" and "Associate" faculty refer to faculty who have separate home departments (or universities) where their FTE line resides.

To give an example: many smaller programs such as Ethnic Studies or LGBT studies may have only one or zero FTE lines, and only consist of associate or affiliate faculty. These faculty might have their home departments in English or Sociology. From an institutional perspective, they are effectively volunteering their time into these other programs.