It's essentially a substance where the mixture has formed a microscopic crystalline structure from the powder that encompasses the other substance (in jellos case, water).
The key to Jello is the crystalline mixture you add to the water has certain properties that when cooled in a specific way cause it to form a crystalline structure that contains the water in a homogeneous way.
*searches on google on what gelatins are made up of*
"Gelatin is a mixture of peptides and proteins produced by partial hydrolysis of collagen extracted from the skin, bones, and connective tissues of animals such as domesticated cattle, chicken, pigs, and fish"
Comments on Profile Post by Wobbly