What does backdoor mean in strategy games like DotA or LoL?

Solution 1:

Backdooring typically refers to the act of taking out an enemies base tower or inhibitor/barracks while the enemy team is slow to respond to your presence, unaware of your advances on their base (due to poor map vision coverage), or otherwise occupied with fighting or pushing on their own.

Generally speaking, in League of Legends, a backdoor will occur while one team is pushing on 5v4, while the last remaining member of the defending team goes to take out towers. Strong backdoor champions include Master Yi (due to his high autoattack damage, steroid skills, and high mobility), Twisted Fate (long range ultimate), Pantheon (long range ultimate), or any other character that excels in dealing auto-attack damage and pushing.

Backdooring is not explicitly disallowed, nor is it a core part of gameplay (meaning you wont see a backdoor happen every game). However, as a rule of thumb, it's generally looked down upon by people in mid to low ELO brackets. You generally wont see it happen as much in higher level play as the enemy will be much more attentive of problem characters like that and wont give them the chance to do so. It's a perfectly legitimate tactic, just difficult to pull off in higher levels of play.

It should be noted that you should not be confusing "backdooring" with "split pushing". Split pushing is when the offensive team attempts to take down two or more towers simultaneously through the use of a single lane distraction. For example, a team may send 3 players to a single lane to push on the tower. The defending team overcompensates and sends all 5 to defend said tower and hopefully secure a few kills. Meanwhile, the other 2 offensive players have full potential to go and destroy another lane's tower.

In both cases, you can utilize long range ultimate characters like Pantheon, Twisted Fate, Nocturne, Shen or Karthus to dissuade or completely stop split pushing / backdooring. Additionally, the summoner spells Teleport and Fortify are great for preventing these tactics. It's not uncommon to have at least one person running Teleport on a team, and in Draft matches (where you have the ability to see the enemy team during champion select), it's often wise to have one character take Fortify if you identify their team as a push team.

Solution 2:

A backdoor is when a single hero goes to attack enemy base while allied minions and enemies aren't there.

Example would be : A team fight is taking place near blue base, one of the five blue heroes avoids the fight to go and attack red towers while red team is occupied with the team fight !

In LOL, good heroes for backdoor are Master Yi, Twisted fate (because of his ult enabling him to back in 2 seconds), Udyr, or any hero dealing good auto-attack damage and that can survive long enough to tower shots.

Solution 3:

Backdoor is when you start attacking enemy tower without your creep wave near that tower. Proof of that is the following.

Backdoor protection in dota 2

From official dota 2 in-game tip

When no enemy creeps are near, Towers and Barracks have Backdoor Protection, which lessens the damage they take and allows them to restore their health.

From wiki:

   All buildings except for the Tier 1 Towers possess the passive
   ability backdoor protection. 

   Backdoor protection activates when there
   are no nearby enemy creeps, and causes the building to take 25%
   damage from illusions and 75% damage from other units. If a
   backdoor-protected building is damaged by an enemy, it will
   regenerate the lost health at a rate of 90 HP per second. It is still
   possible to destroy protected buildings with sufficiently high
   damage. 

   Backdoor protection will not regenerate damage dealt by
   friendly units. On the Tier 2 Towers, backdoor protection is
   independent for each tower. The faction base applies backdoor
   protection as one single unit, e.g. if there are enemy creeps near
   one Tier 3 Tower, all the other Tier 3 towers and barracks will drop
   their backdoor protection as well.

Legality of backdooring along with some other rules depends on the host and the type of the tournament.On official Dota 2 International tournament there were no additional rules against backdooring(except the in-game backdoor protection passive),while on some smaller tournaments there are strict rules about it.

Backdooring was seen in dota 1 as a game guideline(fairplay) not as a major rule.The idea in dota was to help your creeps advance and push towers,rather than heroes just charging towers,and Icefrog wanted to point it out first by game rules later by backdoor protection.

But if Icefrog wanted to make backdooring a serious rule he could simply make it impossible to backdoor,instead he made it just a bit harder to accomplish(especially solo) or in early/midgame.

In public games there are no specific rules about using backdoor,it is only matter of fair play to avoid using it and keep the game fair.On tournaments there could spectator as a referee that follows specific rules and keeps game in line.