For someone who is your mom's brother but not your dad's brother, you would refer to them as your "maternal uncle." "Maternal" indicates that the relationship is through your mother's side of the family.
For a person who is your mother's sister's husband, you would refer to them as your "uncle by marriage" or "maternal uncle-in-law." This term indicates that they are not related by blood, but they are married to your mother's sister, making them a part of your extended family through marriage.