How does a standing army differ from a regular one? Why is a standing army called a standing army

How does a standing army differ from a regular one? Why is a standing army called a standing army and a regular army called regular?

The regular army consists of soldiers who are conscripted for a specific period of service and can also be mobilized in case of war.
The standing army is made up of professional soldiers who are on permanent duty. Differences between a regular army and a permanent one can be displayed in numbers. So the regular army will always be larger than the standing one. An example of a standing army would be Roman legionnaires, who were forced to serve 25 years and were professional fighters.