This refers to any spotting or bleeding that occurs between menstrual cycles and whilst pregnant.
Generally though, we use this term to describe break-through bleeding that occurs while on hormonal contraception. It is more common when you miss taking pills, or are unwell (vomiting, diarrhoea) and the oral hormones are poorly absorbed. It is relatively common in the first six months after a mirena is placed, or with a copper IUD.
Other causes include genital tract infections, cervical ectropion, or as part of early pregnancy.