Refers to a person who indulges in fanciful or unrealistic ideas, often with a slightly negative or romantic connotation.
彼は夢想家で、いつも非現実的な計画ばかり話している。
He is a dreamer, always talking about unrealistic plans.
Compound of 夢想 (musō, 'dream, vision') + 家 (ka, 'person'). The exact historical derivation is uncertain, but the kanji spelling is conventionally associated with the meaning.