What temperature is considered a fever in adults when measured by mouth?

Common knowledge states that a “normal” body temperature is 98.6 degrees Fahrenheit (37 degrees Celsius). However, your baseline body temperature may vary by a degree or more. It also fluctuates. It’s typically lower in the morning and higher in the evening. Healthcare providers often say a fever temperature is 100.4 degrees F (38 degrees C) when measured orally (by mouth). Infants and young children normally have slightly higher body temperatures than older children and adults. So, infants and younger children have slightly higher fever temperatures.

Source: Cleveland Clinic