Most cities it will be something like:
It shall be a by-law offence to discharge an air gun (this includes paintball, pellet, starter pistols, etc.) within city limits except for areas zoned for such activities. Punishable by a $x charge.
Basically it's like any other municipal by-law....
So looking it up for Mississauga the relevant document is here:
Now if you look at the definition of "firearm" technically airsoft is not a firearm (based on the city's definition for the purpose of the document) it still doesn't mean that you should go out and discharge your airsoft gun somewhere. Use common sense.... Remember just because you can doesn't mean you should.
BUT for .177 cal and 4.5mm air pistols/pellet guns they are a "firearm" based on the city's definition so you can't discharge it inside the city and you can be charged up to $5000 for the offence. Now whether or not the police/by law enforcement will charge you is a different story. ie. You may get the pellet guns taken away or charged $50-100, rarely will you ever get charged the full amount.
Now the "shooting range" definition is interesting as it describes an area zoned for such use, so you can go to your local shooting range or a paintball field or whatever to discharge your pellet gun (do not shoot at other paintballers or players) but technically you an discharge your pellet gun at pop cans or w/e at a paintball field because it's "land with or without special equipment being used as of the date of the passing of this by-law for the bone fide discharging of firearms and which discharging would in the absence of this by-law be lawful". So, this is how the cadet forces could shoot pellet guns inside their armoury/building (based on a loose definition).
tl;dr : Don't shoot pellet guns in your backyard. Go to a proper range for that or other area (but do not shoot at other people since these devices can cause bodily injury or death to someone).
PS: This should really have been in the off topic section it doesn't really relate to airsoft.