I think baseline might be one of those words that is open to interpretation within a given company, but it generally represents some sort of standard, whether it is industry accepted or just something that is a requirement of the product.
Does this have to be something measurable? I don't know. In my case, I generally use it related to measurable items, yes, but I suppose it could be something that represents testing using an environment where you expect the product to work, also.
I think Baseline testing is to finalise the functional speciation by reviewing and making a final draft so that it can be used as baseline for creating testcases by testers and to start developement by the developers.