The wiktionary has two different definitions:

1. Any apparently useless activity which, by allowing you to overcome intermediate difficulties, allows you to solve a larger problem.
2. The actually useless activity you do that appears important when you are consciously or unconsciously procrastinating about a larger problem.

This post defines it as:

yak shaving is what you are doing when you're doing some stupid, fiddly little task that bears no obvious relationship to what you're supposed to be working on, but yet a chain of twelve causal relations links what you're doing to the original meta-task.

I personally prefer the final definition because it contains the word "fiddly" which I think perfectly describes a typical yak shaving task.

What is the best definition you've come across or can provide?