noun
fixing agent; fixer
Technical term in photography for a chemical that stabilizes developed images by removing unexposed silver halides.
写真を現像した後、定着剤で処理します。
After developing the photo, we treat it with a fixing agent.
現像液 is the developer that makes the latent image visible, while 定着剤 is the fixer that makes the image permanent.
Compound of 定着 (fixing, adhering) and 剤 (agent, chemical). The term is a direct calque from English 'fixing agent' used in photography.