The term "terminal SNP" is an oxymoron in need of a new, better term. Preferably one with a catchy acronym to encourage adoption. But first, some background for those relatively new to genetic genealogy.
Single nucleotide polymorphisms (SNPs) are mutations that are accumulated sequential between generations. Those on the Y-chromosome, present only in men, are useful in identifying branches in the human evolutionary tree or haplotree. The science is moving fast. As more men are tested, we can identify new and temporarily more recent branches. The most recently identified SNP is often referred to as the "terminal SNP." But obviously, this is only the current state of the science. And, we aspire to find additional branches.
Thus, we need a new term that debunks finality and intrinsically conveys the evolving state of knowledge and our aspirations. Here are some suggestions:
Current terminal SNP: CTS
Most recent known branch: MRKB
Identified distal twig: IDT
Twig awaiting ramification: TAR
TAR seems best. It's easy to say, catchy and conveys the desired set of meanings. It tells us we are at a twig, but also that we aspire to find new branches ... or to further ramify the tree.
So, in my own Y-DNA adventure, I can say my TAR is R-S25738.
It's pretty distal, as the path illustrates,
R-M207 > M173 > L754 > L389 > P297 > M269 > L23 > L51 > L151 > U106 > Z2265 > BY30097 > Z381 > Z301 > L48 > Z9 > Z30 > Z27 > Z345 > Z2 > S15510 > Y7378 > Y7404 > S8958 > S20654 > R-S25738. But already there is a new branch, R-BY19494, but I'm not on it. But there will be a new TAR soon, just for me and my closest relatives.
Perhaps you'll use TAR too ... or come up with another term the community likes better!