A medieval-style version of the IHS (or JHS) monogram of the name of Jesus (i.e. the traditional Christogram symbol of western Christianity), derived from the first three letters of the Greek name of Jesus, Iota-Eta-Sigma (ΙΗΣΟΥΣ).
For a more modern version, see JHS-IHS-Monogram-Name-Jesus.svg.
For a medieval-style IHC (JHC) monogram (alternative lunate-sigma version), see IHC-monogram-Jesus-medievalesque.svg.

SVG version of IHS-monogram-Jesus-medievalesque.png.
Own work -- Converted from the following PostScript code:
AnonMoos