In database management systems, a sequence is an object that generates a sequence of unique numbers or values on demand. A sequence is typically used to generate unique primary key values for a table, but can be used for any application that requires unique numeric values.

In most relational database management systems, a sequence is created using a SQL statement and is stored in the database as an object. The sequence object is defined with a starting value, a minimum value, a maximum value, an increment or decrement value, and a cache size.

Once a sequence is created, it can be used in SQL statements to generate a unique number or value on demand. For example, the NEXTVAL function can be used to get the next value in the sequence, while the CURRVAL function can be used to get the current value of the sequence.

Sequences are useful because they ensure that each row in a table has a unique identifier, which is important for data integrity and consistency. They can also improve database performance by reducing the amount of time needed to generate unique values.