"software as a service" (SaaS; centrally hosted software licensed on a subscription basis) or software interacting with such

Wikipedia describes SaaS as a software licensing and delivery model in which software is licensed on a subscription basis and is centrally hosted. That is, you do not install this software, but rather use it remotely.

For a detailed description, please follow up to that article. For a quick walk on related terms, take a look at this visual explanation:

enter image description here
Terms explained by Pizza: On-Premise, IaaS, PaaS, SaaS (source: Reddit; click image for larger variant)