What are literals?

The terms "literal" and "constant" are synonymous and refer to a fixed data value.

There are:

•Text Literals
Examples: 'JACK', 'BLUE ISLAND', '101'.

•Numeric Literals
Examples: 5001, +256

•Datetime Literals
Example: DATE '1998-12-25'.

Note that the ANSI (American National Standards Institute) date literal contains no time portion.

•Interval Literals
Examples:
INTERVAL '123-2' YEAR(3) TO MONTH
INTERVAL '4 5:12:10.222' DAY TO SECOND(3)


Source: Oracle Documentation