Is there a difference in tepid water and lukewarm water? Are the terms interchangeable?
There is no difference, thus the terms are interchangeable.
Tepid water consists of two parts cold water and one part boiling water, which renders a temperature of about 40 degrees Celsius, which is round about 105 degrees Fahrenheit.
Luke warm water is also considered in the same temperature range, which concludes that both are the same.
A quick test would be that the water to the touch should just be a bit warmer that normal body temperature and should not burn you
In my experience "tepid" means "room temperature" whereas "lukewarm" means "just barely warm". So I'd rate tepid as just slightly cooler. But that's splitting hairs. In practical terms -- ie in recipes -- they're interchangeable.