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The locus of a point 
 (or the envelope of a line) fixed in relation to a curve 
 which slides between fixed curves.
For example, if 
 is a line segment and 
 a point on the line segment, then 
 describes an Ellipse when 
slides so as to touch two Orthogonal straight Lines.  The glissette of the 
Line Segment 
 itself is, in this case, an Astroid.
See also Roulette
References
Besant, W. H.  Notes on Roulettes and Glissettes, 2nd enl. ed.  Cambridge, England: Deighton, Bell & Co., 1890.
 
Lockwood, E. H.  ``Glissettes.''  Ch. 20 in A Book of Curves.  Cambridge, England: Cambridge University Press,
  pp. 160-165, 1967.
 
Yates, R. C.  ``Glissettes.''  A Handbook on Curves and Their Properties.  Ann Arbor, MI: J. W. Edwards, pp. 108-112, 1952.