転呼音

Noun

 * 1)  sound shift
 * 2)  sound shifts from Old Japanese to Modern Japanese, especially those which are preserved in historical kana usage.

Usage notes
In the second sense especially, this term is often translated into English as the truncated version tenko and translated as "change of call," which is a more literal, although slightly misleading, interpretation of the characters ( ten "change/alteration of a word,"  ko "to call").