In ruby what does “$$” mean?

Question

In my rails console:

$$
#=> 12680

in IRB in other tab:

$$
#=> 15550

What does $$ stands fo

Answer 1

I made a wild guess it was can be process it. I validated it with

Process.pid
#=> returns the same value as $$

PS: if there's more to it, please care to add your answer or comment.