involute

Concise Oxford English Dictionary © 2008 Oxford University Press:
involute /ˈɪnvəl(j)uːt/

▶adjective

• 1 formal complicated.

• 2 technical curled spirally.

  ■ Zoology (of a shell) having the whorls wound closely round the axis.

  ■ Botany rolled inwards at the edges.

▶noun Geometry the locus of a point considered as the end of a taut string being unwound from a given curve in the plane of that curve. Compare with evolute.

– origin C17: from L. involutus, past part. of involvere (see involve).