Like guy said their are degrees based on the license. I prefer MIT myself there is also GPL, apache, bsd licensing, which is all technically open source and allow modification. 

The primary difference between those is how attribution and distribution is handled after modification. For instance, in MIT, you don't need to attribute anything you're allowed to do whatever you want. While the others like GPL allow you to modify and distribute something but not charge for it. 

And I know im preaching to the choir here but "source viewable" without the ability to modify is not open source.