The locus of a point which is derived from a fixed point 
by continuous parallel displacement.
Horocycle
References
Coxeter, H. S. M. Introduction to Geometry, 2nd ed. New York: Wiley, p. 300, 1969.Referenced on Wolfram|Alpha
HorocycleCite this as:
Weisstein, Eric W. "Horocycle." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/Horocycle.html