A bit of a segue here and many might not agree, but I think pressure is a state of mind. For example, you can knock in three foot puts on the practice green all day long with no problem, however if you have a three footer to win your club championship suddenly that cup looks a lot smaller and a lot further away. What's changed? It's the same guy as on the practice green, using the same putter, but his perception is now completely different. So while the external variables remain the same, the internal ones go through an upheaval.
So I see pressure as being self-imposed...it's something I work on, in addition to my swing.