What is an acid-base pair?

It is known as conjugate acid-base pair.

When an acid HA dissociates to form H^+ and A^-, the reaction is reversible.
HA ⇌ A^- + H^+
In the forward reaction, HA acts as an acid.
In the backward reaction, A^- acts as a base.
HA and A^- are then known as conjugate acid-base pair.

For example, CH₃COOH ⇌ CH₃COO^- + H^+
CH₃COOH acts as an acid in the forward reaction, and CH₃COO^- acts as a base in the backward reaction.
Therefore, CH₃COOH and CH₃COO^- are said to be conjugate acid-base pair.

In other cases, when a base B accept H^+ to form HB^+, the reaction is reversible.
B + H^+ ⇌ BH^+
In the forward reaction, B acts as a base.
In the backward reaction, BH^+ acts as an acid.
BH^+ and B are then also known as conjugate acid-base pair.

For example, NH₃ + H^+ ⇌ NH₄^+
NH₃ acts as a base in the forward reaction, and NH₄^+ acts as an acid in the backward reaction.
Therefore, NH₄^+ and NH₃ are said to be conjugate acid-base pair.