What is Short-term Exposure Limit?

Legal Definition
A short-term exposure limit (STEL) is the acceptable average exposure over a short period of time, usually 15 minutes as long as the time-weighted average is not exceeded.

STEL is a term used in occupational health, industrial hygiene and toxicology. The STEL may be a legal limit in the United States for exposure of an employee to a chemical substance. The Occupational Safety and Health Administration (U.S. OSHA) has set OSHA-STELs for 1,3-Butadiene, benzene and ethylene oxide. For chemicals, STEL assessments are usually done for 15 minutes and expressed in parts per million (ppm), or sometimes in milligrams per cubic meter (mg/m3).

The American Conference of Governmental Industrial Hygienists publishes a more extensive list of STELs as threshold limit values (TLV-STEL).
-- Wikipedia
Legal Definition
The concentration of a substance in parts per million that a person or body can be exposed to in a defined period of time that is short. It us usually 5 minutes.