I would define backdooring as follows:

Attacking a tower or inhibitor or nexus when all full wave of creeps belonging to the tower or inhibitor or nexus's lane are either moving behind you or engaging with friendly creeps behind you.

Simple and encompasses all backdooring processes. For the purposes of creeps belonging to a tower or inhibitor, assume that the nexus and towers protecting the nexus are owners of all creeps. Thus if any creeps are pushed to the nexus, the nexus isn't being backdoored (Note the above says ALL creeps belonging to the nexus to satisfy).

Thus:

If you attack bottom tower and there are enemy creeps moving behind you, you are backdooring.
If you attack bottom tower and there is a full wave of enemy creeps fighting your creep wave behind you, you are backdooring.
If you attack a tower and there are no enemy creeps behind you, you are not backdooring.
If you attack a tower and only one enemy creep is behind you, you are not backdooring.
If you attack a tower and there are no enemy creeps behind you nor are there any friendly creeps near the enemy tower, you are not backdooring.

Typically speaking, backdooring is accepted by the community but is used as a last resort to try and justify a team's loss to an "unfair yet perfectly fine" tactic.

If you push through bottom tower and then proceed to clear out the entire base, technically your team is "backdooring" the other towers/inhibitors but it's kind of stupid to call it backdooring when you walked into the front door of their base legally. Once you're in, you're in. But if you leave and come back, it's backdooring.