A weak acid is an acid that ionizes only slightly in an aqueous solution. Acetic acid (found in vinegar) is a very common weak acid.

Which is partially ionized in solution?

Weak acids and bases are only partially ionized in their solutions, whereas strong acids and bases are completely ionized when dissolved in water.

What ionizes partially in water?

An acid or base's strength refers to its degree of ionization. A strong acid will completely ionize in water while a weak acid will only partially ionize.