spline

1 :  a thin wood or metal strip used in building construction

2 :  a key that is fixed to one of two connected mechanical parts and fits into a keyway in the other; also :  a keyway for such a key

3 :  a function that is defined on an interval, is used to approximate a given function, and is composed of pieces of simple functions defined on subintervals and joined at their endpoints with a suitable degree of smoothness