An asset of a company that doesn't have any physical existence - but certainly has economic value. Eg, Patents, Know How, Intellectual Property, Goodwill, Branding.
You can't see it, but it's there. It's not material, but it has value. It may not have cost anything, but it's worth something. It's a brand, or a Patent - and it's intangible, but it's an asset all right.