What is the term for a small piece of code that evaluates a value for a single layer property in After Effects?

The term that refers to a small piece of code designed to evaluate a value for a single layer property in After Effects is an expression. Expressions allow users to create dynamic relationships between properties, automate animations, and apply mathematical calculations to layer properties. By writing expressions, you can control values such as position, scale, opacity, and other properties using JavaScript-like syntax, which leads to more complex animations without the need for keyframes for every change.

Using expressions expands the capabilities of After Effects beyond simple keyframing, enabling users to generate procedural animations or link properties of different layers. This enhances workflow efficiency and creativity by allowing for real-time adjustments based on calculations or other layer properties.

In contrast, scripts are larger pieces of code that execute a series of commands in After Effects but are not limited to affecting just one layer property. Animation refers to the process of creating motion and transformations over time within a composition, while presets are pre-defined settings or effects that can be applied to layers or properties but do not involve custom code like expressions do.