In Illinois, maintenance can be terminated if the party receiving maintenance “cohabits with another person on a resident, continuing conjugal basis.” What does that mean? The answer is not as straightforward as it might seem.

https://www.dupagecountydivorcelawyerblog.com/2015/01/25/ter...

Illinois divorce laws are similar. In most cases, spousal support payments will stop if the court finds that someone is living with another person on what’s defined as a “resident, continuing, conjugal basis.” In deciding if a cohabitation arrangement fits this description, the court will look at factors including the nature of the cohabitators’ relationship and the economic impact of the cohabitation arrangements on the ex-spouse who is receiving the support. Note that the sex of a live-in partner is not a factor in this determination.
https://www.hg.org/article.asp?id=26402

http://www.familylegalrights.com/divorce-and-custody-law/whe...
https://illinois-family-lawyer.com/blog/articles/cohabitatio...
http://www.attorneyatlawmagazine.com/chicago/cohabitation-in...

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Note added at 17 hrs (2018-01-09 09:31:40 GMT)
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Establishing Cohabitation
The courts have however, established guidelines which are set forth in 750 ILCS 5/ Illinois Marriage and Dissolution of Marriage Act and in case law. Generally, maintenance in Illinois terminates if someone lives with another person on what is called a “resident, continuing, conjugal basis.” The terms resident and continuing appear clear enough. The term, conjugal generally means marriage- like. Illinois case law addresses the factors which come into play in determining whether a relationship is marriage-like. These factors include whether there is an economic impact due to the relationship and the degree to which the relationship is like a marriage or more of a house sharing arrangement.

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Note added at 17 hrs (2018-01-09 09:35:52 GMT)
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2. As Bill Cosby would have said, “Right. What’s a cohabit?”

a. The supreme court answered the question in 1985 in Sappington. The issue was could there be a “conjugal relation” between a woman and an impotent male. The court said there could be and found cohabitation. This is the first case that began gutting the statute. Now, the need for a sexual relationship.

b. Some of the earlier cases were decided on the basis of whether or not the relationship was continuing. The early cases decided that it was not a continuing relationship if it ended when the petition was filed. The Leming court held there was a temporary cohabitation of several months during which the ex-wife and the boyfriend were engaged to be married but the engagement ended when she moved out of his house and was thus not a reason to end the maintenance. The Second District in toole said that a relationship that ended terminated the right to future maintenance and the right could not be reestablished when the maintenance ended.

c. The third item in the test is “resident.” The early cases said that the living arrangement must be full time and uninterrupted. The Fourth District in Herrin dealt the deathblow to that concept. The Herrin case involved a woman who had her boyfriend spend the night when her children were at their father’s house for his visitation weekends. The Fourth District said that was enough residing together to constitute cohabitation.

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Note added at 17 hrs (2018-01-09 09:37:52 GMT)
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Moving In With a New Boyfriend or Girlfriend Can End Maintenance Payments

If the spouse who is receiving maintenance, or asking for maintenance, moves in with a new boyfriend or girlfriend, this is called cohabitation. If the court finds that the cohabitation existed on a “continuing, conjugal basis,” it will rule that maintenance will not be ordered. In determining whether a party participated in a conjugal relationship, a judge may consider the following factors:

The length of the relationship
Whether the couple shared a home, meals, bank accounts, or household chores
Whether the couple split expenses such as utilities and groceries
Whether the couple vacationed together, spent holidays and birthdays together, or exchanged holiday and birthday gifts
Any other factors a court would find just and equitable to consider

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Note added at 1 day 23 hrs (2018-01-10 15:50:31 GMT) Post-grading
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Non-marital cohabitation (living together): Situation in which two people live together on a stable and continuous basis without this relationship being registered with an authority.
http://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELE...

Wolny związek (kohabitacja lub konkubinat): Sytuacja w której dwie osoby współżyją ze sobą w sposób stały i ciągły jednakże ich związek nie został zarejestrowany przez żaden organ publiczny.
http://eur-lex.europa.eu/legal-content/PL/TXT/?uri=CELEX:520...