noun
A mindset of questioning or not easily accepting claims; often used in contexts like science, philosophy, or personal judgment.
彼の説明には懐疑心を持って聞いたほうがいい。
You should listen to his explanation with skepticism.
科学者には健全な懐疑心が必要だ。
A healthy skepticism is necessary for scientists.
懐疑的 is a na-adjective meaning 'skeptical', describing a person or stance, whereas 懐疑心 is the noun for the skeptical feeling itself.
Compound of 懐疑 (かいぎ, 'doubt, skepticism') and 心 (しん, 'mind, heart'). The exact historical derivation is uncertain, but the word follows a common pattern for abstract nouns expressing mental attitudes.