The term prophylaxis comes from the Greek προφύλαξις, profýlaxis, "guarding or preventing beforehand".
The diagram shows a common opening known as the Sicilian Defense, Najdorf Variation, arising after the moves 1.e4 c5 2.Nf3 d6 3.d4 cxd4 4.Nxd4 Nf6 5.Nc3 a6. Black's fifth move is a prophylactic move that intends to prevent White from placing a knight or bishop on b5.
"Prophylaxis in Chess: A Quick Guide with Examples & Tips". DecodeChess. Retrieved 18 February 2022. https://decodechess.com/prophylaxis-in-chess/ ↩