A tome is a large, heavy book, especially one that's serious or scholarly. When you see a thick history textbook sitting on your desk like a brick, that's a tome. When a fantasy author publishes a 900-page novel that could double as a doorstop, readers might jokingly call it a tome.

The word suggests weight and importance, both literally and figuratively. A slim paperback mystery isn't a tome, but a massive encyclopedia volume is. Libraries are filled with ancient tomes on subjects like philosophy, law, and science. When someone describes a book as a tome, they're usually emphasizing its size and heft, and often its serious subject matter.

You might hear someone say they're “working through a lengthy tome” about World War II, meaning they're reading a substantial, detailed book that takes real time and effort to finish. The word has a slightly humorous or dramatic quality: calling something a tome makes it sound impressively large, perhaps even intimidatingly so.