Englishfor English speakers
quadric
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noun
a curve or surface whose equation (in Cartesian coordinates) is of the second degree
envelope
Noun
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An envelope is a paper cover that you put a letter in to send it.
She put the letter into the envelope, addressed it to Charles, and put a stamp on it.
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An envelope is a covering that goes all the way around something.
Each virus has a special envelope of protein surrounding it.