This is the 357th most frequent Japanese word.
夢
"Dream"
夢 here represents an 'aspiration' or 'goal' that one hopes to achieve.
子供の頃、科学者になる夢を持っていました。
When I was a child, I had a dream of becoming a scientist.
夢 in this context refers to a 'dream' experienced while sleeping.
昨夜見た夢はとても不思議でした。
The dream I had last night was very mysterious.
夢 in this sentence is used metaphorically to describe something 'surreal' or 'fantastical.'