Infarction is death of tissue because the supply of oxygen is cut off. Typically this occurs because a blood vessel is blocked. Causes of blockages include thrombi (clots), bacterial emboli (bits of abscess that break off into the bloodstream), and tumour emboli (bits of tumour which break off into the bloodstream).
Because more than one vessel might supply a tissue an infarction will only occur if i) all the vessels are blocked at once, or ii) the area is supplied by a single artery (also called an ‘end arterial supply’). One organ predisposed to infarction due to having an end arterial supply is the kidney.
At post mortem examination a recent (acute) infarction will be visible as a really well demarcated zone of reddening. This reddening corresponds with the area supplied by the blocked vessel(s).
Over time, the dead tissue is replaced by fibrous tissue (scar tissue) which contracts downwards to leave an indentation.
This kidney has an acute infarction; the dark red wedge on the cranial (top) pole of the kidney. And a more longstanding (chronic) infarction; the indentation on the caudal (bottom) pole of the kidney, where you can see the remnants of the dark red discolouration.