What is a lightweight class?

As far as I know, there is no clear and universally accepted definition of what this term means.

Some people might say that it's a POD (Plain Old Data , where, roughly speaking, the data is arranged in a way that is compatible with the format adopted by C.

Others may say that they are classes with a very short size and can be copied more easily instead of being passed as a reference.

In some contexts there may be a specific explanation.