When it comes to the word balayage, it actually has two distinct meanings. In the world of cosmetics, it refers to a highlighting technique for hair that aims to lighten it in a natural way. In mathematics, on the other hand, it is a method used for reconstructing a harmonic function. Both of these definitions stem from the French word bayalage, which translates to “sweeping”. The cosmetic aspect of balayage is named as such due to the sweeping motion used to apply the color, while the mathematical application involves a mass being “swept out” from a closed domain onto a boundary. The term balayage itself comes from the verb balayer, meaning “to sweep”, which originates from the noun balai meaning “broom”. This can be traced back to either the Old Breton word balan or the Gaulish word balano, both of which also meant “broom”. Interestingly, according to Google NGrams, the usage of balayage peaked in 1974 and has been on the decline since.
