1. A semi-transparent collagen prepared from the air bladders of fish, traditionally sturgeon although other fish are also used.
2. Thin sheets of transparent mica.
3. (colloquial) Sodium silicate ("water glass").
Etymology: from the Dutch word huizenblas (huus, sturgeon, + blase, bladder), first use in the 1500s.
The first definition is the main one, and the only one my childhood dictionary had. In a book I was reading at the time, people had isinglass windows; it was many years before I figured out those were mica windows and not fish windows. ;-)
Isinglass (the fish bladder version) is one of a list of products that can be used as a fining agent (a substance used to remove organic compounds such as yeast) to clarify beer and wine. Other possible fining agents include egg whites, milk powder, gelatin, bentonite, Irish moss, silicon dioxide, chitosan, and PVPP (polyvinylpolypyrolidone).
Not all alcoholic products are clarified, and not all fining agents are animal by-products. But since the fining agents are (completely or almost completely) removed from the end product, you can't always tell if a product is animal-derived just by reading the label.
Sodium silicate dissolved in water has been used to preserve eggs: fresh eggs in the shell are immersed in it to keep out bacteria. Sodium silicate, or the eggs stored in it, can also be used as a fining agent; this may have caused "water glass" to become confused with "isinglass".