By 'sin' the Bible means self-centredness.
God's order is that we love him first,
our neighbour next and
Sin is precisely the reversal of this order.
It is to put ourselves first,
our neighbour next (when it suits our convenience)
and God somewhere in the distant background.
--From "Your Confirmation" (rev. edn. London: Hodder and Stoughton, 1991), p. 21. Thanks John Stott