A mixture of plaster of Paris and glue used to prepare a surface for painting.
1994, Timothy Noad, Patricia Seligman, The Illuminated Alphabet, The Quarto Group (Chartwell Books), 2017, page 27,
The combination of leaf gold and gesso is almost miraculous. No photographic reproduction can adequately show the brilliant effect of raised gesso.
2007, Robin Cormack, Icons, The British Museum Press, Harvard University Press, page 33,
The idea was that this would serve as a binder for the layer of gesso or at least might help to prevent the painting from instantly cracking apart if the wood split at any time.