Semantics: study of linguistic meaning.

Reference: the relationship between a word or expression and the things in the world represented by those words or expressions. A referent is the specific thing that the word or expression refers to.

Sense: the place of an expression in a system of semantic relationships with other expressions in the language. The sense of an expression is its indispensable core of meaning.

On the relationship between sense and reference: the referent of an expression is often a thing or person in the world; whereas the sense of an expression is not a thing at all. In fact, it is difficult to say what sort of entity the sense of an expression is. It is much easier to say whether or not two expressions have the same sense. (Like being able to say that two people are in the same place without being able to say where they are.) The sense of an expression is an abstraction, but it is helpful to note that it is an abstraction that can be entertained in the mind of a language user. When a person understands fully what is said to him, it is reasonable to say that he grasps the sense of the expression he hears.              Semantics: A Coursebook, Hurford & Heasley

Synonymy: the relationship between two expressions that have the same sense. Synonyms, therefore, are expressions which share a sense; perfect synonyms would share all their senses.

EXAMPLES: conceal/hide     purchase/buy     freedom/liberty

Hyponymy: the relationship between expressions such that the meaning of one expression is included in the meaning of the other.

EXAMPLES: scarlet is a hyponym of red.
cat is a hyponym of animal.
oak is a hyponym of tree.

Opposites: the relationship of being "opposite in meaning". There are four basic types of opposites (or incompatibility of meaning):

Complementary (antonyms): expressions which come in pairs and which, between them, exhaust all the relevant possibilities. Being "not X" automatically means being "Y" and being "not Y" means being "X", if X and Y are complementary antonyms.     EXAMPLES: dead/alive     on/off     married/unmarried

(Gradable) Antonyms: expressions are gradable antonyms if they are at opposite ends of a continuous scale of values (a scale which can vary according to the context of use). Thus, with gradable antonyms it is possible to be both "not X" and "not Y", but somewhere in the middle.      EXAMPLES: hot/cold     tall/short     love/hate

(Note: A test for gradability is to see whether the term can combine with something that quantifies it, like very or very much or a little. So one can be "a little hot", but what does it mean to be "a little dead"?)

Converses: expressions which express a relationship between two things such that one of the expressions conveys the relationship in one order and the other expression conveys the relationship in the opposite order.
EXAMPLES: buy/sell     husband (of)/wife (of)     above/below

Non-binary antonyms (incompatible sets): expressions in sets of more then two members which are incompatible in talking about the same thing. All the terms in the given set are incompatible and together all the members of the set cover the entire semantic area.    EXAMPLES: Seasons     liquid/solid/gas     suits of cards

(Note: in this kind of system of X, Y, Z, being "X" means being "not Y" and "not Z"; however, unlike the binary opposition of complementary antonyms, being "not X" does not imply "Y", it implies "Y" or "Z". In other words, something which has a physical state must be either a liquid, a solid, or a gas; if it is a solid, it is neither a liquid nor a gas. If it is not a solid, you know it must be either a liquid or a gas; however, you do not know which one it is merely from knowing it is "not a solid".)

Meronymy: the relationship of a part to a whole. Nose is a meronym of face.