If its not hot or cold then it is warm, but lukewarm would generally be used to describe room temperature to body temperature. I would personally only describe lukewarm as room temperature. (25 degrees Celsius about)

As far as I know, there's nothing in the definition that means it has to be body temperature, though I think that's how it's often used. In my experience, lukewarm implies something is less than warm, or only slightly warm. It is often used negatively, for example 'lukewarm tea' suggests the tea has been left too long and is no longer hot, and 'lukewarm applause' means the applause was half-hearted, as it wasn't as enthusiastic as 'warm applause'.

It literally means "the warmth of luke", so about as warm as Luke would be, body temperature. It's usually used for specific requests: "The water needs to be lukewarm". You use lukewarm instead of warm because "warm" is up to interpretation: if someone just asks for "warm" water, it may come hotter than the requestor wanted!