Looks like the wiper deicer button. You should see heating elements at the base of your windshield (looks similar to those found on the rear window) where the wipers sit. This helps heat up the wiper blades to melt the ice to free your wipers.
That's the Charlie Brown shirt button. Leave any shirt in the trunk, press that button, and it will print the Charlie Brown shirt stripe on it.
I can't think of anything better. You don't want something for the wiper de-icer to be confused with the front defroster nor with any other wiper related icon. The curved panel portion is the accepted image for the windshield among the defroster symbols. The wavy lines are for warm air used(cars had air vents for the rear job originally), so cover the defogging/defrosting action. The wiper de-icer is at the front, but only heating a small portion of the glass. It uses resistant heat, not warm air. The symbol for an electrical resistor is this. Cars with rear wipers can have the defroster wire do that zig zag by the wiper for freeing it.
What do people think the manuals are for? PDFs are more convenient for searching though and readily available from the Toyota owners portal/ The forum is not meant as a replacement for RTFM.
The Prime comes with an inverter? My son's 2023 RAV4 Prime has one too. During extended power outages we have used it to recharge out battery inverter used to power a refrigerator and, occasionally, a freezer.
Top Row: Empty - Auto High Beams* - Wiper De-Icer - Heated Steering Wheel Bottom Row: Fuel Door Release - Power Rear Hatch** - Inverter Switch*** *G, H trims **H trim ***Prime/PHV (as always, manuals are your friend; especially the pictorial index in the front)