From the left 4 dead blog and some good mapping tips.

The dictionary is a set of terms used to describe a space.

1. Close Quarters

Close Quarters are a series of small combat spaces where the survivors must deal with the infected at a close proximity. Typically, the infected can get within 50 feet of the survivors before they are vulnerable. For example, the apartment building at the beginning of the No Mercy campaign in Left 4 Dead is a Close Quarters space. Although the infected have the advantage of appearing at close range, the survivors don't have too many entrances to cover to protect themselves.

Melee weapons in Left 4 Dead 2 add a lot to Close Quarters combat.

2. Narrow Flow

Flow is the main path of the level without taking any detours or dead ends. Narrow Flow is a tight directional space such as a hallway or alleyway that the survivors can cover by only looking forward and backward. If the survivor team has 3 or 4 members active, the first can cover the front, a second can cover the back and the third and fourth members can consider themselves relatively safe. This allows for healing or reloading in a comfortable environment. Of course, the addition of the Charger makes Narrow Flow areas a bit more dangerous.

The Tunnel of Love in the Dark Carnival campaign is an example of Narrow Flow.

3. Wide Flow

Wide Flow is a directional space along the main path as well but is too wide to be safely covered entirely by the front and back. Survivor teams can stick to one side of a Wide Flow area so that they have at least one protected side. Most streets and roads are considered Wide Flow.

This street from The Parish campaign in Left 4 Dead 2 becomes more hazardous with the addition of a car alarm trap.

4. Capillaries

Capillaries are small spaces off the main path that dead end. They can be used for a variety of purposes in Left 4 Dead, such as spawn locations for infected, scavenging of items, providing safe areas for the survivors to duck into during a mob or heal, and to safely avoid a special attack. The best examples of Capillaries are the hospital rooms along the hallways in the No Mercy campaign. Capillaries are also a great place for rescue closets.

Some of the hotel rooms in the first level of Dead Center are capillaries while some are along the main flow. A good strategy is to duck into a capillary when the horde comes.

5. Masking

Masking refers to elements in the world that you can walk and shoot through, but you can't see through. A thick layer of foliage such as trees and bushes are considered Masking. They allow infected to spawn closer in to the survivor team but leave them vulnerable to bullets.

The cane field in Hard Rain is one of the scariest areas in Left 4 Dead 2 - especially in Realism Mode!

6. Sanitizing

When you're playing on the infected team, and the survivors reach a point that is nearly impossible for you to get at them, they may have reached a Sanitizing space.

The CEDA trailer in the 2nd level of The Parish is meant to be a sanitizing space for the survivors to get ready for the run to the bus depot tower.

7. Tube

Lots of times in Left 4 Dead, the main path branches so that the survivors have some choices they get to make. A Tube is a great tool used to create branching pathways. You can go around or travel through the Tube to continue toward your goal. For example, in the subway tunnels in the No Mercy campaign, there's an area with an open subway car that you can go through or choose to go around. Going through the Tube is usually safer for the survivor team.

Survivor teams can get split up exploring this space since they can go around or through the subway car.

8. King of the Hill

Playtests for Left 4 Dead taught us that some of the favorite moments for the survivor team was picking off infected from a higher vantage point. King of the Hill areas allow both the survivor and infected teams to get to a higher elevation.

The rooftops on the way back to the boat in Hard Rain allow an escape from the flooding as well as a better vantage.

9. Fish in a Barrel

Fish in a Barrel is the survivor-safe version of King of the Hill. The infected can't climb up to where the survivors are, but it usually means that the survivors can't get back up to it when they move on.

The fourth map in the Death Toll campaign starts out with a Fish in a Barrel moment where you can clear the area ahead before you get there.

10. One-Way

When a survivor drops into a pit or off of a balcony and he can't climb back up, it's called a One-Way. These are crucial points in VS matches where an infected can grab the last guy to drop down and the survivors are helpless to do much about it. One-Way drops also allow designers to trap the survivor team for finales and mini-finales.

The entire survivor team must coordinate behind the motel in Dark Carnival so that they all drop down the cliff at the same time.

11. Return

A Return is when the survivor team can be returned to an area where they had been before by falling, kind of like a chute in the board game Chutes & Ladders. A Smoker can wait for the right moment and pull a survivor off a bridge or a roof and cause a lot of trouble for the team.

A good smoker can yank a survivor off of this raised walkway forcing the team to split up.

12. Holdout

A Holdout is a defensible position for the survivors in a finale or mini-finale. Sometimes, the Holdout is made more prominent by survivor tools such as propane tanks, gas cans, grenades, and the minigun.

This Holdout space in the generator room of the subway is stocked with health and a minigun.