What is “dimen” in the documentation for the ifthen package?

Question

In the documentation for the ifthen package, there is a test called \lengthtest which compares two <dimen> parameters with the operators <, > or =. (For other test commands, <dimen> is usually <string> or <number>.) Searching for "dimen" in the document and on google, I find nothing to explain what it means.

What is <dimen>?

Answer 1

AFAIK, this would be referring to a length, so, either 1pt, 2cm, 5mm, 10em, etc. or previously defined lengths: \textwidth, \parskip, 4\itemsep, etc.

This would come in handy, for example, if you want to do one thing if the length of your title is larger than the width of the page, and a different thing if it is smaller than it...

Answer 2

Quoting from Donald E. Knuth, The TeXbook, pages 59, 61:

Henceforth we shall use the term <dimen> to stand for a legitimate TeX dimension. [...] [A] complete definition of everything that a <dimen> can be will be given in Chapter 24.

My (hopefully educated) guess is that this is what the ifthen documentation refers to.