The restatement, upwards or downwards, of the value of an asset - tangible and intangible - based upon new circumstances, evidence, or pending sale.
I buy something, anything, and I pay money for it. Later, I realise it's worth an amount different to what I paid originally - so what do I do? Revalue it! If I revalue it upwards, then I've made a profit. Downwards, and I've made a loss.