In human anatomy it can refer to either:

S-1 was a United States government codename for the atomic bomb

S-1 (Muay Thai), the championship of Muay Thai

S1 refers to the "lub" heart sound

S-1 Uranium Committee

A safety phrase, meaning "keep locked up"; see List of S-phrases.

S1 Invicta (car)

S1 MP3 Players

S1 Scout Car, an armoured car built in Australia during the Second World War

Upplands regemente (signal), Signal Regiment of the Swedish Armed Forces

S-1, a block cipher

First year in the Scottish education system

Comet Ikeya-Seki, C/1965 S1

S1 (Munich) S-Bahn line in Munich

S1 (Berlin), a line on the Berlin S-Bahn

Nikon Coolpix S1 digital camera

a microprocessor-core, see S1 Core

SEC Form S-1, used by public companies to register their securities with the U.S. Securities and Exchange Commission (SEC)

ACPI S1 power state in computing

S1 (military), an administrative position within military units.

First sacral vertebra of the vertebral column

S1, Carex elata sedge-swamp, one of the swamp and tall-herb fen communities of the British National Vegetation Classification

S1, an innovative duplex locomotive developed by the Pennsylvania Railroad

S1 (military), an administrative position within military units.

USS S-1 (SS-105), a United States Navy experimental submarine.

S-1 Loening, a United States Army Air Service seaplane

S1 (producer), hip hop producer, member of the group Strange Fruit Project

S-1, a guitar made by the Gibson Guitar Corporation