Dāna: Generosity
Dāna is the open hand behind the cup of cold water, the simple act of giving what another person needs. It stands at the start of the Buddhist path, and it looks to the giver's heart, so water drawn free from a well can carry as much worth as gold.
Cetanā: Intention
The phrase "in the name of a disciple" points to cetanā, the intention that moves the hand before the water is ever poured. The Buddha taught that intention is the root of every deed, so the same cup given because you honor what the disciple stands for weighs far more than a drink handed over out of habit.
Puññakkhetta: Field of merit
A worthy person receives a gift the way good soil receives a seed, and Theravāda calls such a person a puññakkhetta, a field where merit grows. The disciple is that field, so "one of these little ones" turns the tiny cup of water into a seed whose harvest is far larger than itself.
Puñña: Merit
The "reward" Jesus promises is puñña, the store of good that a wholesome deed leaves inside the doer. Merit like this follows the giver everywhere, the way a shadow stays with you in the sun, so the reward is already being carried home the moment the water is poured.
Kamma-vipāka: The ripening of action
"He will in no way lose his reward" names kamma-vipāka, the ripening by which every deed bears its own fruit in time. Cold water given today is like a fruit tree planted in its season: the fruit is sure to come even when it ripens later, which is why Jesus can promise the reward with such firm, trustworthy assurance.