Cost basis
What you paid for an investment, for tax purposes.
Your cost basis is the original amount you paid for an investment, including fees. It is used to work out your gain or loss when you sell.
Why it matters for DCA
Every recurring buy creates a new purchase lot with its own price. Your broker tracks these, and the method used to pick which lots you sell (for example, first in, first out) can change the tax you owe.