What exactly is a Specification?

Question

I read or hear sentences such as:

The Java Persistence API (JPA) is a Java application programming interface specification...

or

Answer 1

If one does a Google search, one gets:

spec·i·fi·ca·tion ˌspesəfəˈkāSH(ə)n/ noun noun: specification; plural noun: specifications

an act of describing or identifying something precisely or of stating a precise requirement.

example: "give a full specification of the job advertised"

(...)

It thus means you describe how something should work, not what one has to do in order to let it work (which is the implementation)

There are formal languages to specify for instance pre- and postconditions of a method. One can then use tools to (semi-)automatically verify a subset of these conditions.

Other formal languages enable you to do automated constraint programming (like for instance my specification to solve a multi-sudoku puzzle).

Another example is CSS: you specify how your webpage should look like, the instructions on how to make a button green are not part of CSS.