In general, synchronous (pronounced SIHN-kro-nuhs, from Greek syn-, meaning "with," and chronos, meaning "time") is an adjective describing objects or events coordinated in time. Examples of synchronous communication include the telephone, chat and instant messaging.

The opposite of synchronous is “asynchronous” with does not require
the events to occur in real time, but rather in a delayed mode.
Examples of asynchronous communications include email, voice mail, discussion boards and course management systems like Blackboard.

What is a Synchronous Course System (SCS)?