Parameter for query

Is there any way to set a variable or parameter for a query?

I have a long query where a certain variable needs to be easy to
change.  I'd like to do something like:

threshold = 10.3

SELECT... WHERE x > $threshold... AND y * 1.3 > $threshold...

Currently, I need to do this in a scripting language.  Is there a
straightforward way to do this directly in Postgres?