Engineering is a max-min problem, finding a maximum of utility at a minimum cost to to client. By broadly expanding cost, client, and utility, that can become "Sustainability Engineering is finding a maximum of social benefit at a minimum negative ecological impact". I'm particularly interested in absolute sustainability at the community scale, and how this definition ties to the concept of Regenerative Design.
From my perspective, there is no concept of sustainability that exists outside of the specific community context (what it takes to be sustainable in Florence would generally not be the same as in Come-by-Chance), so all Sustainability Engineering has to start at the community scale. The challenge, of course, is that it requires multiple scales of community, simultaneously. Water in Toronto, for example, requires municipal, regional, provincial, federal, and international planning and engineering.
The minimum threshold of Community Sustainability would be Needs Security. If you can't engineer to achieve Needs Security, Sustainability is impossible.