In traditional grammar, it is typically defined as a string of words that expresses a complete thought, or as a unit consisting of a subject and predicate.
In non-functional linguistics it is typically defined as a maximal unit of syntactic structure such as a constituent.
This notion contrasts with a curve, which is delimited by phonologic features such as pitch and loudness and markers such as pauses; and with a clause, which is a sequence of words that represents some process going on throughout time.
[1] A sentence can include words grouped meaningfully to express a statement, question, exclamation, request, command, or suggestion.
A clause (simplex) typically contains a predication structure with a subject noun phrase and a finite verb.
An independent clause realises a speech act such as a statement, a question, a command or an offer.
These mostly omit a main verb for the sake of conciseness but may also do so in order to intensify the meaning around the nouns.
[6] The 1980s saw a renewed surge in interest in sentence length, primarily in relation to "other syntactic phenomena".
The textbook Mathematical Linguistics, by András Kornai, suggests that in "journalistic prose the median sentence length is above 15 words".
[12] Research by Erik Schils and Pieter de Haan by sampling five texts showed that two adjacent sentences are more likely to have similar lengths than two non-adjacent sentences, and almost certainly have a similar length when in a work of fiction.
This countered the theory that "authors may aim at an alternation of long and short sentences".