Triangles D E F and D prime E prime F prime are connected at point E. Triangle D E F is rotated about point E to form triangle D prime E prime F prime.

To determine the single rigid transformation required to map triangle \( DEF \) onto triangle \( D'E'F' \), we analyze the given information. Triangle \( DEF \) is rotated about point \( E \) to form triangle \( D'E'F' \).

Since the rotation centers at point \( E \), the transformation involved in mapping \( DEF \) to \( D'E'F' \) is indeed a rotation about point \( E \).

Therefore, the answer is rotation.