Interesting words used in 'The wonder that was India'

I am currently reading The Wonder That was India. The book uses lot of words which I have not come across before –
1) Vassal (Feudatory) – A holder of land by feudal tenure on conditions of homage and allegiance. More on wikipedia
2) Suzerain – A sovereign or state having some control over another state that is internally autonomous
3) Primogeniture – is the right, by law or custom, of the firstborn to inherit the entire estate, to the exclusion of younger siblings
4) Desuetude – the state of being no longer used or practiced. In the book, relates to horse-sacrifice.