So this may be a dumb question, and I apologize in advance. So I was asked today by my principal investigator "what do you call the base immediately next to the terminal base in DNA/RNA?". 

Honestly, I'd just get by using a descriptive sentence like "2nd nucleotide base immediately downstream of 5' end" etc. But he wanted a single word term for it kind of like how we call the last base as the terminal base. 

Couldn't find the answer online so decided to ask here. I don't think he wanted to know to locate the nucleotide base in a sequence since you can use numeric for that. He probably asked in case he had to use a term for it in a paper.