The definition, on several sites, states that it is the 48 contiguous states. That doesn’t mean that Hawaii or Alaska aren’t states, they just aren’t considered when you say “stateside”.
The term stateside was started by GIs during WWII. It meant being in - returning to the US. Being on the (United) States’ side of the Atlantic or Pacific oceans. At that time there were 48 states, so there is a misconception that stateside refers only to the lower 48.
Since then two more states have joined the Union. Stateside, by the original concept, means being in - returning to any and all of the United States of America.
It’s also known, by those in the military, as “The Land of the Big PX.”
For me, stateside equals CONUS (Continental United States) defined variously as either the 48 contiguous states, or the 48 plus Alaska (since it is technically on the continent). The idea being that it excludes holdings and territories. If you served in the military, you were technically on US soil any time you were on base (Guam, for example). However, it made a big difference if the US soil was “stateside.”