Used (often with と) to indicate a noticeably greater degree; even more; much more; further.
彼の英語は去年より一段と上手になった。
His English has become much better than last year.
冬になって一段と寒くなった。
Now that it's winter, it's much colder.
新しいスマホは前のモデルより一段と速い。
The new smartphone is noticeably faster than the previous model.
発表のあとで、彼女は一段と落ち着いて見えた。
After the presentation, she looked considerably calmer.
One step/rung or one level/grade — a single increment (e.g., one stair, one rank, one grade).
階段の一段目で足を踏み外した。
I missed a step on the first stair.
この問題は難易度が上がって一段難しくなった。
The problem became one level harder as the difficulty increased.
彼の給料は今年一段上がった。
His salary rose one grade this year.
One paragraph or passage — a single division of a written text.
その新聞記事の一段目を読んでください。
Please read the first paragraph of that newspaper article.
作文は三つの一段に分けて書きなさい。
Write the essay divided into three paragraphs.
Grammar term (abbreviation for ichidan): a class of verbs (ichidan verbs) whose conjugation involves a single vowel-row change (e.g., miru, taberu).
「食べる」は一段動詞（略して一段）に分類される。
'Taberu' is classified as an ichidan verb (often shortened to 'ichidan').
初級の文法では、まず一段と五段の違いを学ぶ。
In beginner grammar you first learn the difference between ichidan and godan verbs.
「見る」「寝る」は典型的な一段に当たる動詞だ。
'Miru' and 'neru' are typical examples of ichidan verbs.