Static meshes are polygon meshes which constitute a major part of map architecture in Unreal Engine and many other engines like Source. The word "static" refers only to the fact that static meshes can't be vertex animated, as they can be moved, scaled, or reskinned in realtime.

Static Meshes can create more complex shapes than CSG

Constructive solid geometry

Constructive solid geometry is a technique used in solid modeling. Constructive solid geometry allows a modeler to create a complex surface or object by using Boolean operators to combine objects...

 (the other major part of map architecture) and are faster to render per triangle.http://udn.epicgames.com/Two/LevelOptimization They also used to be vertex lit

Gouraud shading

Gouraud shading, named after Henri Gouraud, is an interpolation method used in computer graphics to produce continuous shading of surfaces represented by polygon meshes...

, while CSG is lit per-pixel

Per-pixel lighting

In computer graphics, per-pixel lighting is commonly used to refer to a set of methods for computing illumination at each rendered pixel of an image...

, until the Third generation of the engine, where they also became per-pixel lit.

Characteristics

A Static Mesh contains information about its shape (vertices, edges and sides), a reference to the textures to be used (the actual textures, however, are stored in separate packages), and optionally a collision model (see the simple collision section below).

Collision

There are three ways for a Static Mesh to collide:

• No collision: a static mesh can be set not to block anything. This is often used for small decoration like grass.
• Per-polygon collision (default): individual polygons collide with actors. Each material (i.e. each part of the Static Mesh using a separate texture) can be set to collide or not independently from the rest. The advantage of this method is that one part of the Static Mesh can collide while another doesn't (a common example: a tree's trunk collides, but its leaves don't). The disadvantage is that for complex meshes this can take a lot of processing power.
• Simple collision: the static mesh doesn't collide itself, but has built-in blocking volumes that collide instead. Usually, the blocking volumes will have a simpler shape than the Static Mesh, resulting in faster collision calculation.

Texturing

Although Static Meshes have built-in information on what textures to use, this can be overridden by adding a new skin in the Static Mesh's properties. Alternatively, the Static Mesh itself can be modified to use different textures by default.

Usage

In maps, Static Meshes are very common, as they are used for anything more complex than basic architecture (in which case CSG is used) or terrain (created with a separate TerrainInfo actor).

Additionally, Static Meshes sometimes represent other objects, including weapon projectiles and destroyed vehicles.