Semantics is concerned with meaning; it is a branch within linguistics.
All information found in this post was sourced from the OED.
The word buxom meant "easily bowed or bent; obedient" and now means, "Full of health, vigour, and good temper; well-favoured, plump and comely, ‘jolly’, comfortable-looking (in person). "
As the word progressed through the English language, it resulted in a concrete meaning rather than an abstract meaning. This is an example of narrowing: the meaning gradually turned into a specific, concrete meaning
The word lust meant pleasure during the 9th century, but it now means "sexual appetite or desire; to desire something/someone".
Enthusiasm originally meant, "Possession by a god, supernatural inspiration, prophetic or poetic frenzy; an occasion or manifestation of these." however, it now means, "passionate eagerness in any pursuit, proceeding from an intense conviction of the worthiness of the object."
The semantic change of this word is an example of extension. At first it had a very concrete meaning, but now, it is in a more abstract form.
Horrible meant "Exciting or fitted to excite horror" but now it means "Horribly, terribly; usually as a mere intensive".
This is an example of degeneration. The word transitions into a word of less value (once it meant terror, very specifically, but now it is used in a more vague sense that carries less purpose).