An API or application programming interface is a computing interface exposed by a particular software program, library, operating system or internet service, to allow third parties to use the functionality of that software application. (See: Wikipedia article 'Application programming interface').
Google Trends is a service of Google often used by researchers and market practitioners. It gives a random, statistically representative sample of search term statistics for a given geographical entity. While Google Trends does not have a clearly defined API, it allows programmatic access to its data. CEEMID uses the gtrends R package to access data systematically from Google Trends.