It's specifics, the Kilt was invented as a military uniform, arguably in defense of the rather brutal methods of the Highland Clearances, Kilts were never really Scottish at all, but became Scottish with the tartan re-branding of Scotland that started with Walter Scott. Tartan is also largely a post re-brand invention.
Crofters and people of Highland culture were distinct to Scottish people, even to the point where the reformation didn't touch them and they were wearing Plaids. Plaid is a single bolt of cloth measuring roughly 4-5m long and was worn in any manner of ways for the best part of fifteen hundred years as a form of all in one garment. From around 1600 onwards fashion lent more towards wearing them belted around the waist with the bulk tucked and hanging to the rear so the wearer could wear it over his or her (plaids are non gender specific) body like a cloak.
This gave rise to the "Belted Plaid" which became more and more prevalent towards early 1700's and ubiquitous by 1745 among Highlanders. Around this time the English Army was still trying to be "British" (I've got socks that are older than Britain was then) and after the negative publicity of them ethnically cleansing vast swathes of people since some English toff somewhere decided a human life was worth less than a sheep's, they decided to incorporate something "Scotch" into the Army to make it more inclusive. They decided to make a fancy dress version of the belted plaid and created the skirt, sorry, kilt.
To summerise, Plaid's are ancient and very much like a toga. kilts are a military uniform skirt to appease the masses.
I say all this as someone who lives in the Lowlands, in an area where people spoke Welsh before English, who's last name actually means Welshman, doesn't regard himself as being European and shamefully has worn tartan. A modern one.

