A large portion of the maximum supply being mined during the launch of the cryptocurrency, leading to unfair distribution.
Sometimes this is a result of mining difficulty not re-adjusting as designed and miners being able to find blocks and gain block rewards much faster than expected. Other times it is by design to enrich the founders.
Dash (DASH) underwent an instamine at launch for the first 48 hours. Due to an issue with readjusting difficulty, about 10% of the total supply of Dash was mined within the first 2 days.
* All terms and definitions may update as the Cryptionary improves.