"Attribute: quality, sign or mark which is charatetristic of something or somebody."
In general, a set of attributes is used to characterize a family of objects (a class); for a given object (a class instance), each "attribute" has a specific "value" that is said to be an object "property".
In the context of formal language definition, attributes are associated to grammar symbols (non-terminal, or terminal symbols) to specify the language semantics.
For a particular sentence in that language, the values of the set of attributes associated with the grammar axiom (or root symbol) define its specific meaning.
An attribute should be defined for each data item that should be known about each symbol, after the lexical and syntactical analysis of a sentence in order to do the static semantic analysis and its translation or processing.