Definition:

In general, a group of words that share a specific form or meaning.

More specifically, as defined by John C. Wells (1982), a lexical set is a group of words in which particular vowels are pronounced in the same way.

See also:

• Lexeme
• Phonetics
• Pronunciation
• Semantic Field and Semantic Field Analysis
• Sememe

Etymology:

Introduced by John C. Wells in Accents of English (Cambridge Univ. Press, 1982)

Examples and Observations:

• "The term 'lexical set' . . . was devised by John Wells (1982) as a convenient way of identifying vowel categories not by symbols, but by a set of words in which they occur. Although the vowel in a set like CUP, LUCK, SUN may vary from one variety of English to another, within a given variety there is usually consistency within a set. The lexical set is useful for students who do not have a background in phonetics, since it allows them to identify the sounds involved, even if the symbols for them are not known."
  (Rajend Mesthrie, Introducing Sociolinguistics. Edinburgh Univ. Press, 2000)

• "Though most of modern New Zealand has the [a:] pronunciation of these dance words [sample, demand, plant, branch], it is still somewhat variable for some older speakers, and certainly [æ] was much more common earlier, as confirmed in the commentaries from written records. . . .
  
  "In a letter printed in The Triad (1 Dec. 1909: 7) we read of reactions to the vowels of the BATH lexical set:

  Sir,--Many people, especially those who boast a college education, give such words as grass, brass, castings, class, master, aspect, the absurd pronunciation of grarse, brarse, carstings, clarse, marster, arspect. Why is this thus? . . . [A]ll the above-mentioned words are written in shorthand with the short 'a' not with the 'ah' sound.

  Here we see the stigma attached to the long vowel in the BATH set (represented by the <ar> spellings) in early 1900."
  (Elizabeth Gordon, New Zealand English: Its Origins and Evolution. Cambridge Univ. Press, 2004)