From Out of the Tar Pit:
… the type of complexity we are discussing in this paper is that which makes large systems hard to understand. It is this that causes us to expend huge resources in creating and maintaining such systems. This type of complexity has nothing to do with complexity theory — the branch of computer science which studies the resources consumed by a machine executing a program. The two are completely unrelated — it is a straightforward matter to write a small program in a few lines which is incredibly simple (in our sense) and yet is of the highest complexity class (in the complexity theory sense).