Synonymy

A synonym is a word or phrase that means exactly or nearly the same as another word or phrase in the same language. Words that are synonyms are said to be synonymous, and the state of being a synonym is called synonymy. For example, the words begin, start, commence, and initiate are all synonyms of one another. Words are typically synonymous in one particular sense: for example, long and extended in the context long time or extended time are synonymous, but long cannot be used in the phrase extended family.

Synonymy  is  the  semantic  relationship  that  exists  between  two  (or  more)  words  that  have the same (or nearly the same) meaning and belong to the same part of speech, but are spelled differently. In other words, we can say that synonymy is the semantic equivalence between lexical items. The (pairs of) words that have this kind of semantic relationship are called synonyms, or are said to be synonymous.

big = large
hide= conceal
small = little
couch = sofa
to begin = to start
kind = courteous
beginning = start
to cease = to stop
fast = quickly = rapidly

Pairs  of  words  that  are  synonymous  are  believed  to  share  all  (or  almost  all)  their  se-mantic features or properties. However, no two words have exactly the same meaning in all the  contexts  in  which  they  can  occur.  For  example,  the  verbs  employ  and  use are synony-mous  in  the  expression  We  used/employed  effective  strategies  to  solve  the  problem;  how-ever,  only  use  can  be  used  in  the  following  sentence:  We  used  a  jimmy  bar  to  open  the  door. If we used employ, the sentence would sound awkward *We employed a jimmy bar to open  the  door.  In  short,  we  can  say  that  there  are  no  absolute  synonyms,  i.e.,  pairs  of  words  that  have  the  same  meaning  (or  share  the  same  semantic  features)  in  all  the  situ-ational and syntactic contexts in which they can appear.

(http://webdelprofesor.ula.ve/humanidades/azapata/materias/english_4/unit_1_semantic_relationships.pdf)