What is a turn?

At some point, usually when they were young, most people learned to take turns.  But what is a turn anyway?

In the simplest sense it is "I go, then you go, then I go, and so on..."
Chess has this.  Make a move.  The other player makes their move.  Then you go.

Then there are multi-action single turns, where you may make a move that chains without the opponent being able to respond to the individual steps.
For that, imagine checkers in which you can hop over multiple pieces in a single turn.

Turns may have turns within themselves.  There are macro turns and micro turns.  On the macro level it may be your turn to make the first move, but your opponent can respond in some way, and you can respond to them, yet it is all considered the same turn.
Imagine Magic, in which you may declare attackers, then your opponent casts a spell, which you may then counterspell.  It's all your turn, yet they still make decisions.

A while ago I bought an iPhone game called Dungeon Raid.  It's a match-3 (or more) game layered with an RPG-style upgrading and leveling system.  It uses turns, which are triggered by connecting tiles (coins, monsters, health potions, that sort of thing).  Yet within these turns you can take actions.  I've learned to chain these together into combos that seem to exploit the concept of a turn.

First I use a spell that causes all new tiles to be coins.  Next I use another spell that collects all health potions but converts them to xp, which results in a lot of new coins dropping down.  I follow that with a spell that collects all coins, which are then replaced with more coins.  Finally I convert all coins into monsters.  At this point the turn is not over.  I've collected most of a screen's worth of experience potions and coins.  It is only when I've connected all the monsters to a sword that the game finally counts it as a turn.  Since the new turn hasn't started yet, the new tiles are all coins, so the next turn consists entirely of collecting a ton of coins.

Finally, there is Civilization V.  Against the AI things are pretty clear.  Each player goes in turn, giving build orders, attacking, and initiating diplomacy.  Since one entire side can move in a single turn with no ability for the opponent to react, this makes first strikes excessively powerful, particularly because siege units can attack without taking damage themselves.  The developers may have noticed this and came up with an ingenious solution for multiplayer: players go at the same time.  This lack of turn-taking results in a chaotic mess.  It's like kindergarten all over again and that is why we take turns.  And no pushing.