Companion? Significant other? How we can understand friends who are more than just the 'best'

For many, our life-long "significant other" is a friend. And in a society that obsessed with finding a romantic soul mate, these platonic relationships are left in a gray area.

After forging a strong platonic friendship with a woman named "M," author Rhaina Cohen realized we often lack the words to talk about the variety of life-long best-friendships we can have.

"I would stop at her house on the way to my morning commute and we would go through the grocery store together," Cohen said. "I mean, we were really involved in the day-to-day and also could very much talk about the bigger things that were happening."

Cohen, who authored the recently released book "The Other Significant Others: Reimagining Life with Friendship at the Center," felt excitement and infatuation — the feelings typically associated with budding romantic relationships — despite her relationship with "M" being platonic. At that point, she realized she lacked the vocabulary to describe the nature of their friendship.

Throughout "The Other Significant Others," Cohen explores a variety of relationships that exceed the typical moniker of a friend, but aren't inherently romantic or sexual either.

For example, "passionate friendships," in which friends might cuddle or handhold, or "romantic friendships," which describe many historical relationships between same-sex friends with a level of affection — physical and emotional — that aren't typical today.

"One of my favorite terms that's also in this kind of category is 'smashed,'" said Cohen. "At women's colleges around the early 20th century, when two young women took a shine to each other and became inseparable, they were called 'smashed.'"

Also in the book is an exploration of what's called "compulsive coupledom," the emphasis on and privileging of monogamous marriage that comes with very real societal and legal benefits.

"Marriage went from being a relationship that was very pragmatic to one where people now expect [another] to be their confidant, co-parent, best friend, caregiver, and so on," she said.

By the 19th and 20th centuries, marriage became more about love than the material benefits like a dowry. At the same time, friendship began to decline in significance. The introduction of homosexuality and heterosexuality as identity categories during that time also shifted the broader cultural expectations for romance and friendship.

"You have friendship going from a place where it was really important, one of the most important things of life — often surpassing a spouse — to it becoming this vestige of its former self," Cohen said. "And then marriage being something that takes the place, in the emotional room, that people have in their lives."

Around 45% of the American population is single, meaning that the financial and legal benefits of marriage aren't present in many relationships. Cohen said there are few mechanisms that recognize legitimate authority within a platonic friendship, companion connection, or otherwise similarly strong friendship — though some do exist. For example, in Colorado, a "designated beneficiary agreement" allows friends to bundle rights and responsibilities.

"It's more customizable," Cohen said regarding the range of rights and responsibilities designated beneficiary agreements offer. "I think it would also force a conversation for people about what role they want some significant person to play in their lives, which is not really the case for marriage."

Listen to the full Soundside segment by clicking "play" on the audio icon at the top of this story.