$A(n)$ _______ is a chemical partnership between a weak acid or base and its salt.

Step by Step Solution: Step 1. The blank should be filled with the term 'buffer'. A buffer solution is made up of a weak acid (HA) and its conjugate base (A-) or a weak base (B) and its conjugate acid (BH+). Step 2. In the given statement, the blank refers to a combination of a weak acid or base and its salt, which has the ability to resist changes in pH upon the addition of an acidic or basic substance. This is a key characteristic of a buffer solution. Step 3. Therefore, the term that fills the blank would be 'buffer'. Final Answer: buffer