Across is a preposition and adverb that primarily indicates movement from one side to another side of something.
Common usages of across include:
1. Spatial meaning: "She walked across the street."
2. Figurative meaning: "This idea spread across the country."
3. Mathematical meaning: "3 across 5 equals 15."
4. Idiomatic expressions: "Across the board" (affecting everyone or everything)