Hard cap refers to the maximum supply limit imposed on a cryptocurrency supply. For example, Bitcoin is programmed to produce a maximum of 21 million coins and no further, because the maximum supply is immutable as defined/hardcoded in a cryptocurrency’s blockchain protocol.
However, the more suitable term for the 21 million value is “maximum supply” and the value can not be altered (except due to bugs in the code as in the case of the “Bitcoin 184M” bug).
ICO Hard Caps
During an ICO campaign, an upper limit is set for each round of the campaign at which the round closes, and this is also referred to as a hard cap.
For the hard cap to be reached, the cryptocurrency developers have estimated the amount of funds needed to further the project at the time. They are no longer willing to issue out any more units of the token.