Alternative Pop should be an oxymoron - by one reading, something that is 'alternative' can never also be 'pop', and vice versa. So what Alternative Pop really refers to is music which combines elements from alternative musics and pop, or music which is popular despite using alternative musical styles.
This could mean pop music which is inspired by alternative styles like punk, indie rock or electronica - for example
Avril Lavigne,
Busted or the
Pet Shop Boys. It could also mean alternative music which has pop-like hooks: indie-pop like
The Shins, power pop like
Tegan & Sara, or dance-pop like
Justice.