A great review of the differences and similarities between universal design, inclusive design, and plain old accessibility, from one of the originators of universal design within the web community (as Matt points out the concept's roots go back to architecture in the 1980s, which I hadn't realised).

Some useful definitions:

Accessibility is the goal to ensure that products support each individual user’s needs and preferences. This is a pretty broad definition, I’ll
admit. Notably, it doesn’t define people with disabilities as the beneficiaries, and that’s on purpose.

Universal design is the design of products and environments to be usable by all people, to the greatest extent possible, without the need for adaptation or specialized design.

Matt doesn't explicitly try to define inclusive design, but I like this attempt from OCAD U in Toronto that he highlights:

We have defined Inclusive Design as: design that considers the full range of human diversity with respect to ability, language, culture, gender, age and other forms of human difference.

I also really like this analogy of how you might define climbing a mountain in the context of inclusion:

Inclusive design is the practice of going up the mountain — we can always look for ways to include more people and situations to our designs, even if the result only gets us a few steps up the trail at a time.

Universal design, by contrast, implies that reaching the summit is the true goal. It’s all well and good to talk about inclusion, but if we’re happy enough making it to the first campground up the trail, we’ll never even try to accommodate the effort needed to go all the way.