Open

Open set (mathematics)
Open problem in science and mathematics, a well-defined problem with no known solution.
Open (sport) - A type of competition in tennis and golf (among others) where entry is open to all qualifiers regardless of age.
Open systems
- In science, where an open system (system theory) means a system in which matter or energy can flow into and/or out of the system, in contrast to a closed system in which no energy or matter may enter or leave the system.
- In computing, where an open system (computing) is a computer operating system that provides interoperability, portability or both, particularly Unix systems.
open vowels in Phonetics.
Open... was the original interactive television service on BSkyBs Sky Digital platform.