1. Confirmed by a show of verbal support, rather than by a formal ballot. A: "Was that new regulation passed by ballot?" B: "No, the committee was very vocal in their support for it, so I'm pretty sure it passed by acclamation."
2. Without opposition. The phrase typically applies to elections. Primarily heard in Canada. A: "He won the election by acclamation!" B: "Well, of course he did—he was running opposed!"