Some of the commonly used terms related to the Doom Engine are explained below:
A single point in two-dimensional space. Represented by an X and Y coordinate.
A linedef joins two vertices together. If a linedef is considered as a plane, then it's two sides are the sidedefs. A sidedef is what is visible to the player. If a player can see both sides of a linedef, then the linedef is said to be two-sided (with two sidedefs, left and right). A linedef may also be one-sided where the player is capable of seeing only one side. This must be the right side.
An example of a linedef is a wall that you see in the game. The wall is defined by it's two endpoints. However, you are able to see it's surface also, and what you are seeing is the sidedef.
A sector is defined by a group of three or more linedefs that enclose a certain space that has a single floor height and a single ceiling height. A sector can have any shape, and since neighbouring sectors may be at different heights, an impression of a realistic world is created.
An example of a sector is a room that the player stands in.
An object in the game. Monsters, weapons, keys, and other objects are all "things". A player start position is also a thing and there must be one of these in any level.
Level is a generic term that refers to a single game portion with a well-defined beginning and end. A level begins with the player appearing in one location, moving forward while fighting monsters and solving puzzles, culminating in reaching an end point whereupon the player moves on to the next level.
Textures are the graphics that Doom applies to vertical (walls, etc.) surfaces, or sidedefs. In the game data file, these are Bitmap images. Depending on the way the level is constructed, different textures are applied to sidedefs.
For example, you have walls in Doom that look like they are made from old bricks, you have high-tech walls with wires and lights on them, etc. - all these are just graphic images that are "painted" on the sidedefs.
Flats are very similar to textures, but are applied to horizontal surfaces (floor, ceiling, railing top, etc.) in the sector definition (not in the sidedef definition, as with textures). Like changes in floor/ceiling height, any time you want to make a change in flat you must define a new sector.
Sprites are the technical term for things. A sprite is a graphic image associated with an object. For example, you can see a monster move in the game, and various angles of it are visible depending on the player's position. Thus, each one of these frames is a sprite.
Doom is basically two parts - the EXE file, which contains the game program, that is, the Engine. Doom stores all the data it needs, the sounds, graphics, music, etc., in a file with extension WAD. This stands for Where's All the Data.
Lump is a term used to describe a unit of raw data. This definition, however can be applied to all the components of a WAD file. For example, a single sound is a lump. Also each graphic image - each texture, each flat, each sprite, the title screen, all are lumps.
A level in Doom is stored in the WAD file as a set of 11 lumps.