Technical debt, in its original definition by Ward Cunningham, is the gap between the software that developers built based on an incomplete understanding of the problem. The time making updates to correct the misunderstanding is the technical debt.
In the Manager's Path, Camille Fournier recommends teams:
“...dedicate 20% of your time in every planning session to system sustainability work (“sustainability” instead of the more common “technical debt”).
Sustainability work is a much better term than technical debt. With so many negative connotations about the word "debt", and a tendency to write poor code under the technical debt moniker, the term should be retired.
Technical debt is one of those things that gets pushed into the future until the pain of the debt is so great that the development team pushes for a full rewrite.
Sustainability work instead invokes the craftsmanship behind software development. It pushes the team to close the gap between the system's current state and its true intent.