What does the term "OS rot" refer to?

I'm asking because I just saw a question about the "Best way to avoid OS Rot in Windows 7" and had never heard of the term "OS Rot" before.

It's semi-comprehensible from context in that question, but I thought I'd ask it as a separate question for clarity.

Solution 1:

It's exactly what it sounds like. As you use the OS for a while and add/remove software, things get bloated and feel slower due to things being left around, or just too many things installed in general. Numerous registry entries, files, and even startup entries (which can cause those annoying "not found" messageboxes on startup) are left behind by people who design poor uninstallers for their products. A lot of files (from installed programs and the user's documents) take a while to get indexed as well which can also degrade performance.

Solution 2:

also known as CRUFT

Verity Stob wrote a good article about the different levels of cruft that befall a computer