The definition that is important is that which affects our own behaviour. If to me a picture / story encouraging me to lust, then I need to avoid it. For example I remember a catalogue at work advertising storage boxes which somehow had a boy in the picture. Noone in their right mind would rate it as porn - but for me it was... unhelpful. Similarly in a recent edition of a well known news magazine there was an advert featuring a picture of a couple of boys. Again it 'grabbed' my attention in an unhelpful way. Porn is ultimately in the eye of the beholder.

The biblical test seems to be:

'But among you there must not be even a hint of sexual immorality, or of any kind of impurity, or of greed, because these are improper for God's holy people.' Eph 5 v 3.

On that basis we need to 'flee immorality' (I Cor 6 v 18) and anything that tempts us towards it...