[ak-see-on] 
| a hypothetical particle having no charge, zero spin, and small mass: postulated in some forms of quantum chromodynamics. |
| ax·i·on (āk'sē-ŏn') n. A hypothetical boson having no charge or spin and small mass, proposed to explain the existence of certain symmetries of the strong nuclear force. [axi(al) + -on1.] |
siˌɒn