Haglunds deformity is an anatomic variation in which the back of the heel bone is larger than normal. Generally there is nothing wrong with that except that pressure from the shoes can be painful.
Most Useful Resources:
Haglunds Deformity (Foot Health Forum)
The Haglunds deformity or ‘Pump Bump’ (Foot Health Friday)
Haglunds in an elite marathon runner (Podiatry Arena)
The Pump Bump (Podiatry Ninja)
Haglunds Deformity (Best Running Shoe)