optimism

详细解释:n. 乐观主义, 乐观, 乐天
【医】 乐观主义, 乐观

英语解释：

名词 optimism:

the optimistic feeling that all is going to turn out wella general disposition to expect the best in all things

例句：

Her mood varied from optimism to extreme depression.她的情绪由乐观一变而为极度消沉。