Expressions are SQL-like statements that are used to evaluate dynamic properties.
Expressions can be used to dynamically calculate properties for things like
- filtering features
- generating labels
- generating symbology
They can take into account one or several features with their attributes and geometry as well as layer, project or system specific parameters.
This tag should be used if your question is about the usage of expressions in general like its syntax, special functions, behavior in certain conditions and extension of expression systems.