a movable or folding seat, as in a carriage, taxicab, or limousine, used as an extra seat.
Origin of jump seat
An Americanism dating back to 1860–65
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
a folding seat for temporary use, as on the flight deck of some aircraft for an additional crew member
British a folding seat in a motor vehicle such as in a London taxi
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012