In biochemistry, C1 refers to small molecules with one carbon atom, such as methane, methanol, formaldehyde and formate, carbon dioxide, and carbon monoxide.

 %C1 is the Url Encoded version of the Acute accent for A (Á)

Counter-Strike "C1", a slang expression from the video game, used to express agreement

C0 and C1 control codes defined in ISO 6429

In town and country planning in the UK, C1 is the code for permission to use specific land or premises for hotels

C1 Corvette, any model corvette sold from 1953 to 1962, denoted a solid-axle rearend

C1 is an abbreviation for single-person Sprint Canoe.

USS Newark (C-1), a protected cruiser of the United States Navy

ISO 216 C1, an international standard paper size (648×917 mm)

BMW C1, a security scooter with a roof

C1 is a level in the Common European Framework of Reference for Languages

TCSEC C1 security class

Kawasaki C-1, an indigenous transport aircraft of the Japanese Air Self-Defense Forces

Ford C1 platform, the Ford Motor Company's global compact car automobile platform

USS C-1 (SS-9), a C-class submarine of the United States Navy

Douglas C-1, a cargo/transport airplane produced by Douglas Aircraft for the U.S. Army Air Service starting in 1925

Citroën C1, a small car

C1 (protocol), a protocol for file transfer, also known as "Punter"

C-One single-board microcomputer

NRS social grade C1 for the United Kingdom is the lower middle class

AEG C.I, a German World War I reconnaissance aircraft

HMS C1, a C-class submarine of the Royal Navy

Cytochrome C1, a protein, precursor to Cytochrome C

C1, a British designation for a cargo/transport aircraft

C-1 Trader, a cargo-version of the S-2 Tracker anti-submarine warfare aircraft of the United States Navy

'C1' was the top category in Group C sportscar racing, used between 1985 and 1990

ATI C1, codename of the Xbox 360 GPU "Xenos"

In football, C1 refers to the UEFA Champions League (formerly known as the European Cup)

First cervical vertebra of the vertebral column, in human anatomy

In mathematics, the notation <math>C^1</math> refers to the set of all continuously differentiable functions

C1 Television, a Mongolian Television Channel

A unit of the South African Police responsible for assassinating opponents of Apartheid during the 1980s (also known as Vlakplaas).