Set of technologies used to realise projects. Most of the time, a technology is closely linked with a specific language.
Open-source relational database management system (RDBMS).
Open Computing Language - an open standard for high-performance heterogeneous programming. OpenCL is in most part an API, but it is also a programming language that is based on C99. It is a simplified version of C99, so projects involving OpenCL are in a sense projects involving C.
Open Graphics Library. It is a widely supported and popular standard in computer graphics. See https://en.wikipedia.org/wiki/OpenGL for more information.