Acid-base equilibria refer to the balance between acids and bases in a solution, which affects the pH level. Acids are substances that donate protons (H⁺ ions), while bases accept protons. The strength of an acid or base is determined by its ability to dissociate in water, forming ions. This equilibrium is crucial in various chemical reactions and biological processes. The Brønsted-Lowry theory defines acids and bases based on proton transfer, while the Lewis theory expands this definition to include electron pair donation and acceptance. Understanding these equilibria helps in fields like chemistry, biology, and environmental science.