1. [adj]  used of syllables; "an atonic syllable carries no stress"
2. [adj]  characterized by a lack of tonus

ATONIC is a 6 letter word that starts with A.

1. \A*ton"ic\, a. [Cf. F. atonique. See {Atony}.]
   1. (Med.) Characterized by atony, or want of vital energy;
      as, an atonic disease.
   2. (Gram.) Unaccented; as, an atonic syllable.
   3. Destitute of tone vocality; surd. --Rush.
   

2. \A*ton"ic\, n.
   1. (Gram.) A word that has no accent.
   2. An element of speech entirely destitute of vocality, or
      produced by the breath alone; a nonvocal or surd
      consonant; a breathing. --Rush.
   3. (Med.) A remedy capable of allaying organic excitement or
      irritation. --Dunglison.