chapter one; first chapter
Used in books, reports, or any structured document to refer to the initial chapter.
第一章では、問題の背景を説明します。
In chapter one, we explain the background of the problem.
Standard kanji spelling using the numeral 一.
Variant using the full-width digit １; common in digital text and tables.
Compound of 第 (ordinal prefix) + 一 (one) + 章 (chapter).