If something is geodesic, in the way learners usually meet the word, it has a roundish shape built from many small flat faces — mostly triangles, sometimes pentagons — fitted together so the whole thing curves, like a football or a dome. A geodesic dome is the famous example: a light, strong hall or greenhouse made of a triangle net. Though actually, scientists first used geodesic for the shortest path you can draw on a curved surface (the way a plane’s route looks bent on a flat map but is “straight” on the globe). The dome is named from that idea: its bars follow those short curves. You will hear the adjective most often with “dome.” It is a specialist word — building design, maths, science museums — not daily talk.