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（恶劣环境中的）宜人之地，乐土；（不愉快情形下的）开心之时，乐事
You can refer to a pleasant place or situation as an oasis when it is surrounded by unpleasant ones.
例句
The immaculately tended gardens are an oasis in the midst of Cairo's urban sprawl.
在开罗不断扩张的都市版图内，这些被悉心照料的花园成了一片世外桃源。