The term “All-Time High” relates to the highest price that an asset has achieved on an exchange, for the current trading pair that is being referenced. For example, if a share of stock in XZY Corp comes to IPO at a price of $5 per share, then trades as high as $20 per share, before falling to $10 in a certain period of time, we could say that the “All-Time High” for the XZY Corp share price was $20.
The ATH value represents the theoretical maximum price that one could have sold the particular asset for, and also represents the maximum price that another trader was willing to pay for that asset, during that period. However, given the fractional nature of most digital assets, it is possible that the ATH was derived through the trade of a fraction of an asset, rather than a full coin or token.
For example, during the height of a bull-run, a trader may make a purchase of 0.1BTC for $5,000 just before a big drop. Proportionally speaking, this would give Bitcoin a new ATH at the price of $50,000 per unit of BTC, although only 0.1 BTC ever traded at that price.
The concept of All-Time High may also be applied to the values of market capitalization (market cap). In early January 2018 - a couple of weeks after Bitcoin’s ATH - the market cap of the entire cryptocurrency market reached an ATH of about $661.2 billion.
The opposite of ATH is the “All Time Low” (ATL) which is used to refer to the lowest price point an asset has traded at, typically only recorded after an asset is listed and begins trading on an exchange.