cavalry

noun

1. (historical, uncountable, usually)The military arm of service that fights while riding horses.

   “Two hundred heavily-armed cavalry rode behind the Audley standard, while close at their heels came the Duke of Lancaster with a glittering train, heralds tabarded with the royal arms riding three deep upon cream-colored chargers in front of him.”

2. (countable)An individual unit of this arm of service.
3. (countable)The branch of the military transported by fast light vehicles, also known as mechanized cavalry.
4. (countable, figuratively, uncountable)A source of rescue, especially in an emergency.

   “call in the cavalry”

   “"We landed in their killbox?" "We did. I am sor... No, belay that. Heads down! Cavalry incoming!"”