Varnish is a solution of oils or resinous substances in a solvent, which volatilizes or dries in air by evaporation of solvents or the action of a catalyst, leaving a layer or film on the surface to which it has been applied. There are varnishes of natural origin, generally derived from resins and essential oils of plants, and synthetic varnishes.

Its application to wood and other surfaces has the primary objective of preserving them from the action of atmospheric agents if it is exposed to the outside or of protecting and giving beauty as well as physical and chemical resistance if its destination is interior. It can accept dyes or colorants that modify its color and tone. Over the years, wood finishes have found many characteristics in varnishes not possessed by shellac; for example: its ability to harden when dry, its ability to acquire a great shine when polished, rubber effect, craquelé, textured, and a certain degree of waterproofing against water, moisture or grease. Currently, there are instant-drying, low-maintenance varnishes, resistant to UVA rays and abrasion, for both wooden floors and furniture and a wide variety of surfaces.[1]​.

Varnish, according to the Dictionary of the Spanish Language and other manuals of the Spanish language, comes from the dialectal word "berniz", and this is from the low Latin veronix veronicis, in relation to the resin sandáraca, from the Egyptian city of Beronice. Other sources cite Berenice, the name of the ancient Greek colony of present-day Benghazi in Libya. The Latin construction per (intensive) and nitére ('shine') is also proposed.[2]​ It accepts synonyms such as dye, enamel and luster, and concomitances with glaze, lacquer, patent leather, patina "Pátina (copper)"), maque (lacquer) and mogate (potter's varnish).[3]​.

The Egyptians already used varnishes to decorate their tombs, and the Greeks to protect the wood of their ships against the corrosive power of sea salts, but its origin seems more likely in India, China or Japan (where the art of varnishing has been dated half a millennium BC and certainly from the 3rd century, as a technique of Korean origin). Other sources consider that China and India knew about varnishes long before Japanese lacquer craftsmen used them.

Varnishes can be grouped according to the resin or material that composes them, or according to the solvent used.