A group of artists in Scotland who were active around the 1880s. They rebelled against what they considered the conservative and creatively stifling influence of the Royal Academy. They were very much in favour of outdoor painting as opposed to long indoor studio work.
The confusing part about the term Glasgow school is that it also applies to a second wave of Scottish artists (the Second Glasgow School) as well as being a term more loosely associated with early 20th century painters who developed their very own kind of art nouveau for which they are now best known.
You can also find mention of the Glasgow School in this article on Art Nouveau