That's a lot of questions - I'll tackle just the first one... Both "optimum feasibility" and "optimal feasibility" generally sound fine - the semantic difference is miniscule - the choice is mainly determined by style preference, but there may be other subtle factors at work, so additional context could be useful here, for example a complete sentence - maybe even surrounding sentences.

"Optimal" and "optimum" are often interchangeable, but "optimal" is more commonly used before nouns (e.g., optimal feasibility). Glossaries at the end of books help clarify terms, mainly for instructional purposes and to avoid ambiguity. Writers may also define words in their own terms to make concepts clearer for readers.

The words "optimum", "maximum", and "minimum" are derived from Latin nouns and work best as nouns. If you want an adjective, "optimal", "maximal", and "minimal" are better. The roles can be reversed, of course. ALL adjectives can be repurposed as adjectives so long as the context is clear. It seems to me that scientists are especially prone to use "-um" words as nouns. Mathematicians often use "maximum", "minimum", and "optimum" that way. Here are some other words scientists use to create nouns: "datum", "quantum", "consortium", "forum". "Quantum" is an especially good example. It is the word that physicists use to name a "quantum" of energy. It can be used, of course, as an adjective too, as in "Quantum Physics", but even there it feels more like a noun that is dressed up as an adjective.

"Optimal" vs. "Optimum" – Both are correct, but "optimal" is more commonly used as an adjective (e.g., "optimal feasibility"). "Optimum" can be both a noun ("the optimum") and an adjective, but "optimal" sounds more natural in technical or formal writing when describing a condition or quality. * ✅ "We aim for optimal feasibility." (preferred) * ✅ "We aim to reach the optimum." (less common as an adjective) Glossaries in books – Glossaries are typically included to clarify terms for instructional purposes and to avoid ambiguity, especially in technical or academic texts. They help ensure that readers have a consistent understanding of key terms. Defining words in their own terms – Yes, writers often define terms in their own words to enhance understanding, especially if the concept is complex or used in a specific context. This helps make the material more accessible to readers.

Other Dan covered the first question, so as to glossaries . . . . I suppose there may be several reasons authors do this. Even though you could just look up words in a dictionary, they may want to put the words you are likely to need in one convenient location. Also, they will want to specify which definition of each word is appropriate to their book. Other times, there may be unusual terms that would be difficult for a person to look up on their own.