Dirt speck
It is probably a speck of dust on the sensor. Get a blower from any camera shop, remove the lens,lock up the mirror (mirror lock up in camera menu) and give the sensor a good blow ( without touching it) while holding the camera front down so that any specks of dust fall away from the sensor. Sometimes specks can stick (pollen for example) in which case you could try a disposable swab and cleaning solution, although if unsure most camera shops provide the service pretty cheap - the sensor would be a lot more expensive to replace if you mess it up.
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