Sargability
The property of a query predicate being expressed in a form that can efficiently use an index or another selective access path.
An indexed column does not guarantee an index-friendly predicate. Wrapping the column in a function, placing an implicit conversion on the wrong side, or expressing a range indirectly can change the optimizer's available access paths.
Sargability is therefore less a rule that says “use the index” than a relationship between predicate form and searchable access paths. The engineering meaning of relational indexes and Oracle SQL performance provide the broader context.