According to the Illinois instructions:
An Illinois resident is an individual that is domiciled in Illinois for the entire tax year.
A Nonresident of Illinois is an individual that is not domiciled in Illinois at any time during the year.
A Part-Year Resident is an individual that moved into or out of Illinois during the year.
If one spouse is a resident and the other is not, you may file separate state returns. If you file a joint Illinois return, both spouses will be taxed as if they are residents. To file separate returns within your account, you can file the federal return and wait for the return to be accepted. Then, change the filing status for the primary taxpayer and file a separate return for the first spouse. The second spouse will need to create a new account and file the separate state return in that account (if required to file).
For more information, see: