What is sponsored content?

If it walks like a duck and quacks like a duck, is it sponsored content?

In a comment on the Shape of Outbounding thread I mentioned a desire to, in the least, have sponsored content marked as such.

But what is sponsored content?

Here are some cases of what I'd consider to be sponsored content along with an example:

1) Company X pays Y to write about Z.

Costa Rica DMO pays Sally Blogger to write about national parks in Costa Rica. This is really PR, but, well, all PR is sponsored content.

2) Company X gives Y a free trip with the expectation they'll write about Z.

Costa Rica DMO gives Sally Blogger a free trip to write about national parks in Costa Rica.

3) Hotel A gives X a free room on the expectation of a review.

Costa Rica Excelsior Palace Hotel comps a room to Sally Blogger in return for a review.

The above are all pretty clear cut (to my mind) as there's a direct (theoretical at least) financial benefit for company X or the hotel which has a direct relationship with the writer and there is an expectation they will deliver copy. All require a disclosure (IMO).

But what about if there isn't. For example:

a) Retainer

Jeans company Z pays Sally a travel stipend to travel where they want, as they want, they should say thanks now and then. Gives them free jeans. Is everything Sally writes sponsored by Z Jeans? Should it be disclosed? What if Z Jeans was an airline or a hotel chain?

2) Travel/insurance/clothing etc affiliates

Using my own website as an example, we have plenty of affiliate links to Agoda (an Asia-focused OTA) on accommodation reviews on our site. These are all (in theory!) income earning links, but the writer isn't involved in matching those up and the hotel generally doesn't know we're writing a review. But if you draw a bow long enough, you could argue Agoda paying me a commission allows me to pay the writer who is responsible for reviewing the hotel. Is that sponsored content?

Am sure there's lots of other examples and what ifs. Be interested to hear what others think.

[Late edit post David's comment: Just to note I didn't bother with the obvious bodgy link buying/selling that would fall under this. Just wanted to stick with the less obvious area addressed above.]