First, it's important to recognize that "sustainability" is a noun and "sustainable" is an adjective. Sustainability n. 1. the ability to be sustained, supported, upheld, or confirmed. 2. Environmental Science. the quality of not being harmful to the environment or depleting natural resources, and thereby supporting long-term ecological balance, e.g. The committee is developing sustainability standards for products that use energy. So, in your example: - a sustainability partner is a person or company or organization that has agreed to work with their other partners to sustain (maintain/keep in good condition) the environment. 'sustainability partners' are two nouns like 'football team' or 'government department'. - a sustainable partner is a partner that we hope will 'stay/be kept/remain as a partner', just like two people who get married... we hope the husband and wife can stay together.

"Sustainability Partner" is a term referring to businesses that help other businesses with environmental issues. Without more context, I don't know that I can give a better definition. However, it is definitely not the same as sustainable or a long term partner.

It means that it can maintain, withstand or endure something. Depending on the context the definition of the word varies a little but overall maintains the same purpose. To sustain something it has to be durable. So for example, 'long-term partner' would be someone who can endure the hard times for the sake of continuing a relationship. Sorry, I think you're referring to relationships :P