system
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human
What does "m" in atomic mass number mean?

I sat down here reading and I don't seem to understand what "m" means in the ending of an atomic mass number. For an example the isotope 234m Protactinium. First of all what does it mean? And how would it affect decay?
gpt
It denotes isomeric states. These are nuclear excited states which have long half-lives. 

When the nucleus is in a metastable excited state it will eventually decay, either to the ground state of the same nucleus via gamma emission or internal conversion, or it will just decay directly into some other nucleus by alpha, beta, or whatever.