What type of account is the accumulated depreciation account?

It is categorized as a contra account.

Essentially, it reduces the amount shown on the balance sheet for Property, Plant & Equipment (PP&E), to account for the depreciation that has accumulated over the years that the company has owned the assets.

Generally, most companies show PP&E on their balance sheet as a net amount, meaning: ( Total Book Value of Assets - Accumulated Amortization )