A sexual arousal over fire, either in seeing one, talking about one, or in actually setting one.
The main difference between Pyrophiliac's and Pyromaniac's is that a Pyrophiliac derives a sexual pleasure, arousal, which supposedly the Pyromaniac doesn't.
Many mainstream Shrinks don't quite accept this difference.
It is also considered an 'uncommon' Paraphilia, and that one so afflicted, does not necessarily derive their sexual arousal from the actual setting and/or watching of fires.
The mere talking about it, the fantasizing of it, apparently can be enough for arousal, along with masturbation aids.