Look no further... in the same book there's is a list of Acronyms & other terms in Appendix C:
HIPER
High Impact Pervasive. An APAR that
describes a serious problem or one
that has a wide spread impact. The
APAR should be reviewed and if
applicable, the PTF should be
installed as soon as possible.
NoHIPER - not a general/widespread/serious problem... You can run the system safely without it - install this fix at your earliest convenience.
HTH,
p5wizard