open system

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A physical system that interacts with and accepts input from other systems, especially one that obeys conservation laws.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun computing A system allowing hardware and software from different manufacturers to be used together seamlessly.
• noun physics A system that can exchange both matter and energy with its surroundings.