Seva is work done without expectation of return — not because reward is scorned but because the doer is out of the way. The Bhagavad Gita‘s Karma Yoga is seva systematized: do the work; release the fruit.
In Sikhism, the langar — the free community kitchen attached to every gurdwara — is the most visible seva. Anyone may eat, anyone may serve. The ethic extends outward: the hands, the money, the attention given without accounting.