a horizontal bar on a typewriter keyboard that is depressed in order to resume typing one space to the right.
a horizontal bar on a computer keyboard that is depressed in order to move the cursor one space to the right or, often, to perform some function in a program.
Origin of space bar
First recorded in 1885–90
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019
British Dictionary definitions for space-bar
a horizontal bar on a typewriter that is depressed in order to leave a space between words, letters, etc
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