What does backdoor technically mean in strategy games like DotA or League of Legends? Is it an accepted tactic or there are some rules that forbid it?
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.
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
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.
Backdooring means "attacking an enemy tower without the presence of allied minions" that is all it is. The number of allies or enemies present is irrelevant.
As for rules and "acceptance". Different games deal with it in different ways. DotA used to give towers regeneration if no minions were present so that you couldnt chip away at towers. LoL gives towers added armor and magic resist to make them tougher. These are the "in-game" mechanics to balance backdooring. For individual tournaments they may impliment different rules regarding the tactic. That being said, you need to kill towers to get to the enemy nexus so why not take one out when you have the opportunity? It is part of the game and the designers have addressed the matter in a manner they deem sufficient.
The definition of backdooring has changed over time.
The original usage was the act of attacking towers that were not the most exterior. For example, I would attack your base towers, barracks, or throne while you still had a complete set of outer towers standing. Changes in game mechanics have since made this impossible: you now must kill the most exterior tower in a lane, then the next tower, before moving on to the base tower in that lane.
IceFrog currently refers to backdooring as beginning an attack on towers without allied creeps present. Because he felt this was a problem, he has implemented extra defenses for towers when this occurs: they will take less damage and regenerate quickly to the health they were at prior to the backdooring attempt, if not killed off completely.
Backdooring in competitive play has in the past been explicitly banned in some tournaments, but rarely if at all in DotA since the above change was implemented. In HoN, it is still common for tournaments to ban backdooring base towers and Barracks, because neither the above change nor something which would address the problem have been added to the game.
Backdooring is thus, in high-level DotA, considered an acceptable tactic, typically countered through the use of TP scrolls and/or the structure fortification ability. Backdooring in HoN is still rarely seen but can be problematic. Opinion is split on whether this is acceptable or counterable. I do not know what the stance of LoL players is on the issue.
This is not the official terminology, but I see backdoor as way to take down the enemy base without going there by normal walking.
So in this definition you are backdooring when:
Thus, you avoid any kind of detection the enemy could have place to control the map. This would better suit the original meaning of the word. Note that even with that definition, I find it a perfectly legal way of playing. At the end, this is war and you want to win no matter what.
To go to the real definition, you can see here that it is when the enemy (one or more) attack one of your structure without support of their minions.
Backdooring has been endorsed as a acceptable tactic by Riot. And by the way, when you attack a tower without the support of your minions, turrets take only half damage from champions. So the rule is: if you can sustain the damage, you can backdoor.
Many players tend to reject that technique because of BDing being forbidden on the time of DotA (due to imbalance in some heroes capabilities). But as far as LoL is concerned backdooring is not a banable technique.
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.