From the second edition (1989):
terminal element, f. dia)lect, used to designate a regional or social variety within a language as in idiolect; also used in forming a number of technical terms in linguistics, as acrolect, basilect, isolect, sociolect, etc. (see quots.). Hence (without hyphen) as n., a social variety of a language or dialect.
1965 W. A. Stewart in R. W. Shuy Social Dial. & Lang. Learning 15, I will refer to this topmost dialect in the local sociolinguistic hierarchy as acrolect (from acro- ‘apex’ plus -lect as in dialect). In most cases what is meant by ‘Standard’ English is either acrolect or something close to it. At the other extreme is a kind of speech which I refer to hereafter as basilect (from basi- ‘bottom’). 1969 Florida FL Reporter VII. i. 48 Although acrolect differs also in sounds and words from basilect, grammatical differences between them create the real blocks to communication. 1971 C.-J. N. Bailey in Working Papers in Ling. (Univ. of Hawaii) III. v. 39 In this case, the creole becomes a satellite (satellect or acolutholect) to the established language (matrilect). Ibid., The matrilect serves as the acrolect in the continuum at one end, while the basilect‥will be separated from the acrolect by a graded (systematic) series of mesolects. Ibid., An isolect has been defined by me elsewhere as a form of speech different from its isolectal correlate. Ibid. 41, I have suggested paralect to denote folk creations from the related systems (e.g. middle Arabic, Punti, Slavish). 1972 J. L. Dillard Black English iii. 107 Higginson recorded many clause and question forms which are much as they still are in Black English basilect today. Ibid. 300 Dialect refers to a set of features delimited geographically; sociolect to a socially distributed set. 1974 J. Nist Handicapped Eng. iii. 72 Any departure from that code‥marks the speaker as confined to either the vernacular of mesilect or to the ‘folk speech’ of basilect. Ibid., Geographical dialects in present-day British English automatically become social~class lects. 1975 College Composition & Communication XXVI. i. 104/1 She is being primed to be the ideal teacher of basilect students in spite of her messy lect.