The highest point (in price, in market capitalization) that a cryptocurrency has been in history. *see All-Time-Low (ATL).
"All-Time High" (ATH) refers to the highest price (or market cap) that an asset has reached since its listing or inception. As the price used to define the "all-time high" is the last done, it just refers to the highest price a trader paid for an asset, regardless of how much he bought of the asset.
The all-time high is often quoted in financial assets globally, as it shows the theoretical potential" of the asset. That said, there is no guarantee that the asset will trade back at its all-time highs again, and this metric is more often quoted as a vanity metric.