Historical Linguistics/Semantic Change

Just as the pronunciation of words change over a language history, so do their meanings and connotations. Unlike sound changes and other highly regular processes, semantic change is entirely unsystematic: the change of any one word is independent and generally unaffected by the changes of other words.

Influences of Phonetic Similarity
Words which sound similar will converge or become more similar in meaning.
 * eg. Obnoxious once meant "exposed to injury," but now means "annoying, offensive, or objectionable" - a change motivated by the words resemblance to "noxious"
 * eg. Some speakers use "bemused" and "amused" interchangeably.

Widening
A word comes to refer to a more general set of things than its earlier use.

Leeched Diminutives
A word frequently used as a diminutive changes to refer to the thing it describes generally
 * A subset of widening
 * eg. New High German Mädchen 'girl' was originally a diminutive of Magd 'girl'

Narrowing
A broad category comes to refer to a specific element
 * eg. corn, meaning grain, shifted to refer specifically to maize.
 * eg. meat, meaning food, shifted to refer specifically to food made from animal flesh.

Pejorization amd Meliorization
Respectively, when a word comes to refer to positive or negative instances of its base meaning.
 * eg. dom, meaning judgement, became doom, meaning peril, a pejorization.

Semantic Bleaching
An intensifier becomes weaker in meaning.
 * eg. terribly once meant "so extreme as to be terrifying" whereas now one might say "I terribly glad to see you."

Euphemism
Taboo terms are replaced with other existing words.