In climbing, to "send" a route means to climb it cleanly from bottom to top — no falls, no resting on the rope, no aid. The verb is unique to climbing slang and has been adopted everywhere: "I sent my project," "did you send?", "send the finish."
Sending standards differ slightly by discipline. In bouldering, a send means topping out the problem in a single attempt, hitting every move without falling. In sport climbing, it means leading the route while clipping every bolt without weighting the rope or pulling on gear. In trad climbing, the same definition applies plus placing all protection clean on lead. In every discipline, falling once anywhere on the route disqualifies the attempt — it becomes a "go" or a "burn," not a send.
The word probably comes from "ascend" via 1980s American climbing slang. It captures something specific to the sport: the combination of effort, mental commitment, and the binary feeling of finally completing a route after multiple attempts. "Send it!" has become the universal climbing encouragement.