Shutter lag usually means the time between pressing the shutter button and the moment the exposure actually begins.

It is not just the mirror movement in a DSLR. Mirror and shutter mechanics can be part of it, but the total delay may also include camera processing, metering, and sometimes focus acquisition depending on how the term is being used.

That’s why the term can be a little ambiguous:

• In a strict sense, shutter lag often means the shortest possible press-to-exposure delay, assuming focus and metering are already locked or pre-computed.
• In a broader sense, people may use it to describe the full delay they experience after pressing the button, including autofocus and metering time.

Autofocus lag is more specific: it’s the delay caused by the camera needing to achieve focus before taking the shot.

So your understanding is partly right, but shutter lag is generally more than just mirror delay. The most useful practical definition is simply “how long it takes from pressing the shutter to the photo being taken.”