There are a couple of definitions of airlock. One is of particular interest to astronauts who go on spacewalks and submariners who go on … I don’t know what they are called. Are they sea walks? Undersea walks? Whatever. Them.
An airlock is a compartment with two doors, both of which are airtight when closed.
If you don’t know what airtight means, be patient. It’s coming up in about a dozen or so entries. If I explain airlock here I’ll have nothing to say when I get to it, so you’ll have to wait. However, if you make a guess as to its meaning based on the components of the word, you’ll probably get it right.
One door of the airlock leads into the spaceship, regular ship or submarine, as the case may be. The other door leads to space in the case of a spaceship or sea in the case of a submarine or an airlock positioned below the waterline in a regular ship. At most, only one of those doors will be open at a time.
The idea is that someone can leave the ship (whatever kind it is) without flooding a sea-based ship or letting out all of the air of a space-based ship. To do so, the person opens the door from the ship, enters the airlock, closes the door, opens the outer door, and leaves. On his or her return he or she enters the airlock (opening the outer door first if it was closed), closes the outer door, waits for the water to be pumped out on sea-based ships or air to be pumped in on space-based ships, and then opens the inner door and returns to the main section of the ship.
I don’t know if astronauts or submariners feel the way I do about this concept, but after a spacewalk or a subsea walk, I’d want a drink. In fact, as far as I’m concerned, it would be best if they keep a wine supply in the air lock for that purpose. I don’t want to wait any longer than necessary.