妥当（だとう） means reasonable, valid, or appropriate based on logic. It sounds fairly formal and is common in business, reports, analysis, law, and academic writing.
Use it when something can be defended as sensible or valid, not just when you personally like it. Common pairings include 妥当な金額（だとうなきんがく） for a reasonable amount/price and 妥当な判断（だとうなはんだん） for a sound judgment.
The noun 妥当性（だとうせい） means validity or reasonableness, often used when checking whether an idea, method, result, or data set holds up.
Compare 適切（てきせつ）, which means suitable for the situation or the right fit. 妥当 focuses more on whether something is logically acceptable or defensible among possible choices.
As a verb, 妥当する（だとうする） is more technical and means to apply or to be valid in a case. For everyday “is reasonable,” use 妥当だ（だとうだ） instead.