The ` key which is to the left of the number 1 on a standard english keyboard.

If the squadron selected is ready to launch, they will launch.

If the squadron selected is already launched, they will return.

On top of what Benser said....

If selected squadron is not ready to launch, you can also press ` key to prepare that squadron to launch after you designate how many planes you want to launch on that squadron (So it's like clicking on Ready button on GUI). However, Beware on that pressing ` while launch is being prepared but not completed will cancel the process (again, in this case, it's like clicking on Cancel). Happens sometimes, especially when I want fighters out ASAP as I see oncoming hoard of clustered DB. 

Want to add something;
I've got a Norwegian keyboard, and were unable to use the hotkey, so If you got anything other than an english keyboard, you may not be able to use the hotkey (left of "1") unless you change the keyboard language to English. 

Thank

Since no-one mentioned its actual name:

Tilde(~) Key

On the standard American 101/102 keyboard layout. Its located:
Below Escape(Esc)
Left of One (1 !)
Above Tab 


Or on older keyboards:
Left of the Enter Key. 



Key Functions in NavyFIELD:
Launch Aircraft (Only if Select Group is loaded.)
Cancel Aircraft Loading (Only if Selected Group is loading.)
Initiate Selected Aircraft Total Loading (Only if group has no assigned planes and a number of planes has been selected.)
Set Selected Air Group to Return (Only if group is airborne.)

Ya what normpearii said :) 

But not every keyboard layout got the tilde accessable to use ingame (Like my norwegian keyboard) 
Only work around I've got is to add a secondary keyboard language so you can change the layout in-game (Left+L-alt+L-shift I belive)

Check this out if you don't know how to do it.
Hope it helps :)  

I changed my hot key to something else, because I find myself accidentally clicking it a lot.  And it will return a squad of planes, or cancel the loading of them.  It's because I'm a noob cv driver though haha.  I prefer to just click launch them.