The word "underseat" refers to the space or area underneath a seat, typically in a vehicle or on a mode of transportation such as a plane or train. Full definition
Refrigerated / Cooled Box Located In The Glovebox, Driver / Passenger And Rear Door Bins and 1st Row Underseat Storage (mercedesbenzgreenwich.com)
The large underseat basket with access from the front is roomy enough for all your baby gear and belongings. (albeebaby.com)
SR5 Package (2 - Box Over-Head Console, Clear Front Fog & Driving Lights, Power Sliding Rear Window w / Privacy Glass, Rear Underseat Storage Tray, and SR5 Badge), Black w / Perforated Leather Seat Trim, ABS brakes, Front dual zone A / C, Front reading lights, Heated door mirrors, Low tire pressure warning, Outside temperature display, Overhead console, and Remote keyless entry. (smartmotorguide.com)