Cloud IP PBX |

a Cloud (or Hosted) Private Branch Exchange (PBX) is "a telephone exchange system built, delivered and managed by a third-party service provider. Hosted PBX is an IP-based telephony solution provisioned and accessed entirely through the Internet."

There's a variety of similar definitions all over the internet, but to keep it simple: Cloud PBX is a virtual phone system that provides secure and reliable business phone system through the internet.