1. Any apparently useless activity which, by allowing you to overcome intermediate difficulties, allows you to solve a larger problem. I was doing a bit of yak shaving this morning, and it looks like it might have paid off.

2. A less useful activity done consciously or subconsciously to procrastinate about a larger but more useful task.

I had thought that only definition 1 counted as Yak Shaving but apparently definition 2 is also in common usage.

I would personally find the concept more useful if it only meant definition 1. But that might just be me.