Apart from power and current, which signal property can amplifiers be specifically designed to increase?

Amplifiers are electronic devices designed to increase the magnitude of a specific signal property. The three primary types of amplification in electronics focus on voltage, current, and power. Since power and current are already mentioned in the question, voltage is the missing fundamental property. Distractors like frequency and phase are properties of the signal that a standard linear amplifier is specifically designed to preserve, not increase. Linearity is a performance characteristic of the amplifier itself, not a signal property that gets amplified.