If the term lukewarm is in reference to a liquid like milk or water for a recipe, then this temperature is anywhere between 98 to 105 degrees Fahrenheit. This temperature also is comparable to combining one part of water that is at room temperature with two parts of boiling water. When preparing milk formula, lukewarm is usually the ideal temperature range.
As a cooking term, the definition of lukewarm is given as the temperature that is near one's body temperature. The word 'lukewarm' comes from the Middle English word "luke" that meant tepid. The use of the word "lukewarm" with the meaning of slightly warm began about the 14th century.