In 1877, the United States created a common-law marriage. The definition of this is a couple who has lived together for a certain amount of time and presents themselves to the community as a married couple. Basically, they qualify for some of the legal benefits of marriage without all the paperwork!
To qualify for a common-law marriage couples must:
Be legal age and of sound mind
Intend to marry one another at some point
Refer to each other as your partner, spouse, husband, or wife.
Not every state in the United States recognized common law marriage, and since you're asking this question, you may have figured out that Nevada is one of the states who don't.
What they do recognize is something called the "cohabitation agreement." This agreement allows unmarried couples to share property, but do not receive any other legal benefits awarded to couples who officially tie the knot.
This article 'Common-law partner agreement' is super helpful in navigating the laws and regulations surrounding unofficial marriages.