A script used to set a property value for a layer in an After Effects composition.
Expressions have read access to all the properties of any layer, composition or source in the project, as well as being able to sample pixel values for layers and compositions. They are not however, able to change the pixels of a composition directly, or alter any property apart from the property to which they are applied.
Expressions are calculated with each frame (or more accurately every time the composition is rendered, which can be many times per frame with motion blur) and are unable to store values between frames.
Expressions should not be confused with the Extendscript scripting framework, which can be used to control the application itself.