[ape4]

Wikipedia has a reasonable definition: a concept in software development that reflects the implied cost of additional rework caused by choosing an easy (limited) solution now instead of using a better approach that would take longer.[2]

https://en.wikipedia.org/wiki/Technical_debt

[AnIdiotOnTheNet]

I would disagree in that I would say it isn't necessarily the 'easy' or 'limited' solutions, but basically any solution that, in retrospect, is now a limiting factor in your development. Overengineering is just as bad as under if it later limits your performance and/or flexibility.