A medium is a pourable runny product that is designed to be added to paint to extend the colour, working time, or change the product to make it more or less glossy, 2-tone(interference mica) or perhaps alter it to be fabric friendly. Glazes, as top coats also fall into this category, as do tinted glazes for scumble effects, both dark and light on top of texture.
In this video Leandra explores a wide variety of mediums, and gives tips and idea how these are applicable to mixed media crafting.
Links:
Crackle Glaze Video https://youtu.be/Bk8a8IFAkuI
Topic 15 Blog Intro: Gels and Mediums http://blog.paperartsy.co.uk/2015/08/2015-topic-15-gels-and-mediums-challenge.html
Gels/ Mediums Masterclass Blog Post (this post shows close up samples of items shown in this video) http://blog.paperartsy.co.uk/2015/08/2015-15-masterclass-by-leandra-franich.html
PaperArtsy Shop http://shop.paperartsy.co.uk/gels-pastes-texture-rust--metal-effects-284-c.asp