1. (of a problem) That which continues over an extended period of time, often associated with suffering ("long suffering").
2. (medicine) Prolonged or slow to heal. The opposite of acute.
3. (medicine) A condition of extended duration, either continuous or marked by frequent recurrence. Sometimes implies a worsening condition (a condition which gets worse with each recurrence), though this definition is not inherent in the term.
4. Inveterate or habitual.
a chronic smoker
5. (informal) Very bad, awful.
6. (informal) Extremely serious.
they left him in a chronic condition
http://en.wiktionary.org/wiki/chronic