Open

• Wiktionary:Open - definition
• Open set (mathematics)
• 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.