A proper ideal I of a ring R is called semiprime if, whenever J^n subset I for an ideal J of R and some positive integer, then J subset I. In other words, the quotient ring R/I is a semiprime ring.
If R is a commutative ring, this is equivalent to requiring that I coincides with its radical (and in this case I is also called an ideal radical). This means that, whenever a certain positive integer power x^n of an element x of R belongs to I, the element x itself lies in I. A prime ideal is certainly semiprime, but the latter is a strictly more general notion. The ideal of the ring of integers Z is not prime, but it is semiprime, since for all integers a, a^n is a multiple of 6=2·3 iff a is, since both 2 and 3 must appear in its prime factorization. The same argument shows that the ideal of Z is always semiprime if n is squarefree. This is not necessarily the case when n is a semiprime number, which causes a conflict in terminology.
In general, the semiprime ideals of a principal ideal domain are the proper ideals whose generator has no multiple prime factors.