1. The first book in the His Dark Materials series by Philip Pullman.
2. An object of the same name or alethiometer (literally Truth-Measure) within said book. It is used by Lyra (the protaganist) to find truth and meaning throughout her journey. It resembeles a large golden-colored pocketwatch with three knobs on it's rim. It opens to reveal an ornate circle with 36 distinct symbols at it's edge with three large hands and one small hand which move around it. Each symbol has a series of meanings which, when held in one's mind may pose as a question or an answer. The big hands move to form your question with the knobs and the little hand moves to give the answer.
- Extract from The Golden Compass (aka Northern Lights)