In ruby what does “$$” mean?

Question

In my rails console:

$$
#=> 12680

in IRB in other tab:

$$
#=> 15550

What does $$ stands fo

Answer 1

My google-fu shows me that $$ is the process number of the Ruby running the script.

Answer 2

From "Ruby Programming/Syntax/Variables and Constants":

$$ - $PROCESS_ID, $PID or Process.pid - The process number of the Ruby running this script.

Answer 3

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.