Definitions of tonic:

• noun:   a medicine that strengthens and invigorates
• noun:   (music) the first note of a diatonic scale
• noun:   lime- or lemon-flavored carbonated water containing quinine
• noun:   a sweet drink containing carbonated water and flavoring

  Example: "In New England they call sodas tonics"

• adjective:   used of syllables

  Example: "A tonic syllables carries the main stress in a word"

• adjective:   relating to or being the keynote of a major or minor scale

  Example: "Tonic harmony"

• adjective:   employing variations in pitch to distinguish meanings of otherwise similar words
• adjective:   of or relating to or producing normal tone or tonus in muscles or tissue

  Example: "A tonic reflex"

• adjective:   imparting vitality and energy
• name:  A surname (very rare: popularity rank in the U.S.: #67098)