What is an "asynchronous logger"?

[sketch34]

I can take a guess but the documentation doesn't define it (that I could find so far). I want to know the details of what this means in the context of spdlog.

[tt4g]

Sink will process log messages in a dedicated thread.
Synchronous loggers block the calling thread until the I/O is finished when sink writes a log message to a file, but asynchronous loggers do not block.

See Wiki for more details: https://github.com/gabime/spdlog/wiki/6.-Asynchronous-logging