In traditional terms, the mother of your spouse is commonly referred to as your "mother-in-law." This term is used regardless of whether you and your spouse have children together. The title of "mother-in-law" is a way to acknowledge the familial relationship that exists due to marriage.
However, it's essential to recognize that family dynamics and terminologies can vary across cultures and personal preferences. Some people might choose to use different terms or titles to address their in-laws based on their unique circumstances or cultural norms. For example, they might use affectionate nicknames or simply refer to their mother-in-law by her first name if that feels more comfortable for everyone involved.
Ultimately, the best approach is to communicate openly and respectfully with your spouse and the mother-in-law about how you both feel comfortable addressing each other. Respecting each other's preferences and feelings is key to maintaining a healthy and positive relationship with your extended family.