'Yearly' or 'annual' means that something takes place each year : 'We have an annual audit' , 'We do a yearly stock-take'. per annum [abbreviated 'pa'] means 'in a year' and is used for quantities, in particular salaries : 'The salary will be £25,000 pa.' 'There has been an increase of 12% pa in sales'. 'per year' means the same as 'per annum'