what is an axillary fever?

Be
I only have an armpit/oral thermometer. My son feels hot, so I left him undressed under the fan for a bit and took his temp. It was 99.0 under his arm. I'm waiting a bit and will take it again. He also has a rash down his upper back and on his face and head. I was told that was normal, but with this temperature is it? Also, is that temp even considered a fever? I'm probably going to take him in regardless because he's only 6 weeks and preterm at that. ☹️