The phrase gets used loosely enough to be meaningless. Some companies mean a longer visit. Some mean the same visit with a bigger number attached. Here is the honest distinction.
A one-time clean maintains a state
It assumes the house was clean recently and returns it to that. Surfaces get wiped, floors get done, bathrooms get reset. Nothing is dismantled, because nothing needs to be.
A deep clean changes the state
Vent covers come off. The range hood filter goes in the sink. Baseboards get washed rather than dusted. That is a real difference, because dusting a baseboard that has six months of kitchen grease on it just relocates the grease.
Cabinets get emptied at the areas that matter, the fridge comes apart down to the crisper runners, and the oven gets taken properly apart rather than given a wipe of the door glass.
How to tell which one you need
If you cannot remember the last time anything was moved, it is a deep clean. If you are looking at one specific thing that has gotten away from you, like the oven or the shower glass, that might just be an add-on to a normal visit, and we will tell you so rather than upselling you.