Thermal adhesive is a type of thermally conductive glue used for electronic components and heatsinks. It can be available as a paste (similar to thermal paste) or as a double-sided tape.
It is commonly used to bond heatsinks to integrated circuits where there are no other mounting mechanisms available.
The glue is typically a two-part epoxy resin (usually for paste products) or cyanoacrylate (for tapes). The thermally conductive material can vary including metals, metal oxides, silica or ceramic microspheres. The latter are found in products that have much higher dielectric strength, although this comes at the cost of lower thermal conductivity.
End-user modding heatsinks may come with thermal (tape) adhesive attached, but for products sold through electronic components distributors this is hardly ever the case, i.e. the adhesives are sold separately to professionals.