There really isn't an appropriate synonym and the word is so easily misunderstood.
DeYoung says it means that God
cannot suffer and is incapable of being acted upon by an external force.
This teaching is based on the Bible's assertion that God is unchangeable and sovereign.
DeYoung carefully articulates it so that it doesn't make God out to be passive, uncaring or unresponsive to our needs.
Although the idea that God suffers is popular today, he points out that if God is changeable, this means that he is not dependable and can't be trusted.
It is well worth reading the whole article, typos and all, to appreciate the point and value of this teaching. Phil Johnson's article God without mood swings is also worth chewing over.
I found it hard to swallow this teaching at first, because those who were arguing for it presented little Scripture in support and seemed to be explaining away large tracts of Scripture that didn't fit with the teaching. But DeYoung doesn't do that.
Thanks for helping me to understand what everyone else was going on about, Kev.