Jargon - Attack Surface - What is it?

Everyone sprinkles these words to make their argument sound good.

So, I thought I will find a simple article that could explain it 

According to the article:- 

There are three basic interrelated considerations that develop from our examples:

1. Network Attack Surface, the attack will often be delivered via a network
2. Software Attack Surface, with a primary focus on web applications
3. Human Attack Surface, social engineering, errors, trusted insider, death and disease

The links below has more information:-

http://www.sans.edu/research/security-laboratory/article/did-attack-surface