Stéphane Beaulieu offers a concise summary of the scholarly concept of "Deutero-Isaiah" in discussions of authorship.
Stéphane Beaulieu, “Deutero-Isaiah,” in The Lexham Bible Dictionary, ed. John D. Barry et al. (Bellingham, WA: Lexham Press, 2016) (Logos ed.)
DEUTERO-ISAIAH (or “Second Isaiah”). A section of the biblical book of Isaiah variously identified by modern scholars as Isa 40–55 or Isa 40–66.
The three-author view subdivides the book of Isaiah into three units, each written by a different author:
• First Isaiah = Isa 1–39
• Second Isaiah = Isa 40–55, calling this section Deutero-Isaiah
• Third Isaiah = Isa 56–66
The two-author view limits Isaiah to two divisions:
• First Isaiah = Isa 1–39
• Second Isaiah = Isa 40–66, calling this section Deutero-Isaiah
Thus, at times scholars use Deutero-Isaiah synonymously with Second Isaiah (with both meaning Isa 40–66), while other scholars use the term to refer only to Isa 40–55. Most scholars use the terms First Isaiah, Deutero-Isaiah (or Second Isaiah), and Third Isaiah to refer to the authors of sections of Isaiah, but at times the terms are also used just to refer to the major literary sections without any implication of a multi-authorship viewpoint.