I think the most popular definition would be that the government stands ready to enforce private contracts and prevent fraud, but makes no regulations about what is a valid contract between informed and consenting parties.
The difficulty is in dealing with externalities. All arguments about 'limiting' capitalism or disallowing certain types of contracts are really about what causes externalities and what doesn't, or alternatively, how bad that externality is.
Depending on people's personal views, there's all kinds of arguments anyone could make - that's the tricky bit. For example, I can say that no one should be allowed to wear yellow because people who wear yellow will go to hell. Additionally, I believe that them wearing it will encourage others to as well. So my claim is that you choosing to wear yellow doesn't just impose a cost on you, so you can't say, 'Oh, well, I don't care if I'm going to hell - that's my choice' because I'm claiming that there's a cost bourn by others that you're not taking into account when you wear it.
So, perhaps, I lobby the government to impose a fine. I believe that a fine of £X will be sufficient disincentive for other people to follow your lead. Now you're fully internalizing that cost.
All economic arguments eventually can be understood in this framework; from abortion, to wealth redistribution, to banking regulation, to smoking, to prostitution, to parking tickets, etc...Even arguments against capitalism as a whole make use of the capitalistic framework in their arguments. I've never heard a communist say, 'Oh, I think we'll make a ton more stuff and be richer under communism.' They say, 'the worker is separated from his labor by those who control the means of production.' In other words, the current system isn't taking into account the full cost bourn by the capitalistic system.
Funny that, eh?
(EDIT: By the way, terms like 'means of production' aren't real things. These are Marxist concepts that exist in philosophy, but have no proven value or definition. Which is also interesting, because although they are Marxist terms, they presuppose the concept of ownership.)
(EDIT part deux: Sorry about using the term 'externality' as it's a specific economic specialist word, but I couldn't think of a simple corollary. I looked at the Wikipedia page and it's really good: http://en.wikipedia.org/wiki/Externality . You probably get it, though, from the context in the 'yellow' example.)
(EDIT C: (Sorry) 'Service sector' is production as well. There's no difference between producing something that you can touch and something that's in your head. Wealth is created either way. The compensation people receive is relative to the reduction in entropy of the their local area. Read: If people want it, it's wealth.)