The IRS calls the sum of the employer contributions and your contributions through a payroll deduction plan to be "employer contributions".

It's counter-intuitive, but the reason is because this sum (which is reported in box 12 with a code "W" on your W-2) is treated the same way - both amounts are pre-tax, and the employer places both amounts into the HSA. Another way to look at it is that you and your employer negotiated a payroll deduction plan, so that the employer paid part of your salary to the HSA in return for you not having to pay tax on it.

This is as opposed to amounts that you might contribute to your HSA independent of your employer (which is possible).