# Why was 'structural' chosen to mean 'structural unemployment'?

Source: Economics (3 ed, 2014) by N Gregory Mankiw, Mark P. Taylor

structural unemployment [=] unemployment that results because the number of jobs available in some labour markets is insufficient to provide a job for everyone who wants one

This definition (below) doesn't answer my question; all unemployment is caused by structure of the economy. So what's so structural about structural unemployment?
1. c. : involved in or caused by structure[,] especially of the economy