+1(514) 937-9445 or Toll-free (Canada & US) +1 (888) 947-9445
in what context?

To become a resident is kind of obvious :)

Cease to be a resident - you gave up residency, or failed to maintain it.
 
For immigration purpose, like Baloo said;

For income tax purpose, see this for reference

http://www.canadavisa.com/canada-immigration-discussion-board/am-i-required-to-file-tax-returns-t66717.0.html