Final answer:
An acid is a substance that releases hydrogen ions (H+) in water, and the definition includes both strong acids, which fully ionize in solution, and weak acids, which do not.
Step-by-step explanation:
The simplest definition of an acid is that it is a substance that releases hydrogen ions (H+) in water. When an acid dissolves in water, it dissociates to produce these ions, which are simply protons (since a hydrogen atom consists of one proton and one electron). According to the Arrhenius definition, an acid increases the amount of hydrogen ions in an aqueous solution, while the Brønsted-Lowry definition describes acids as proton donors. Additionally, strong acids completely ionize in solution, releasing all of their hydrogen ions, whereas weak acids do not fully ionize, meaning some hydrogen ions remain bonded within the compound while in solution.