Webster's English Dictionary

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envelope

en.ve.lope \'en-v*-.lo-p, 'a:n-\ n 1: something that envelops : WRAPPER  2: 
   a flat usu. paper container (as for a letter)  3a: the outer covering of an 
   aerostat  3b: the bag containing the gas in a balloon or airship  4: a 
   natural enclosing covering (as a membrane)  5a: a curve tangent to each of 
   a family of curves  5b: a surface tangent to each of a family of surfaces