(1) A button on a keyboard.
(2) In database management systems, a key is a field that you use to sort data. It can also be called a key field , sort key, index, or key word. For example, if you sort records by age, then the age field is a key. Most database management systems allow you to have more than one key so that you can sort records in different ways. One of the keys is designated the primary key, and must hold a unique value for each record. A key field that identifies records in a different table is called a foreign key.
(3) A password or table needed to decipher encoded data.