Sam Altman has recently expressed interest in seeing more 'high-tech' submissions to YC http://blog.samaltman.com/new-rfs-breakthrough-technologies. I agree that defining 'high-tech' vs 'tech' is a bit ambiguous and fraught with potential disagreement, however for the purposes here, we are talking about companies that engage in some type of basic R&D in pursuit of the core product development.

'Science' is a grouping used to categorize funding from the National Science Foundation (NSF) - cross-functional, but a good enough bucket for the purposes of the key used.