柔軟剤 is fabric softener. Formation: 柔軟 'soft, pliable' + 剤 'agent, preparation' — the same suffix as in 洗剤 and 漂白剤.
It goes into a separate compartment from the detergent and is released during the rinse, so it is not mixed in at the start of the wash.
Collocations
- 柔軟剤を入れる: to add fabric softener
- 柔軟剤の香り: the scent of fabric softener — a heavily marketed selling point in Japan
- 無香料の柔軟剤: unscented fabric softener