A white hat is an ethical security hacker. Ethical hacking is a term meant to imply a broader category than just penetration testing. Under the owner's consent, white hat hackers aim to identify any vulnerabilities the current system has.
The word "hacker" conjures the image of someone with ill intent toward individuals, websites, and company information systems. The prevailing theory is they look for ways to mine company data and destroy or change customer information. Those types of "bad guys" certainly exist — the cybersecurity industry calls them Black Hats, but in reality, they aren't the only hackers lurking in cyberspace.