Highest historic price an asset has been traded at.
The All Time High usually refers to a price held for at least a short period (Perhaps 5 minutes?) across multiple exchanges; Not just a single momentary spike on a single exchange. The price of a cryptocurrency can shift drastically in short term spikes on isolated exchanges if they are dealing with liquidity issues or spikes in volume.
If these swings are isolated the true price will usually be correctly quickly via trader arbitrage.
* All terms and definitions may update as the Cryptionary improves.