The term "adult nit" refers to a lice egg that has fully matured into a viable head louse. Some common synonyms for this term include "mature nit," "developed nit," and "grown nit." These terms all refer to the same thing: an egg that has hatched and developed into a fully formed louse. Other synonyms might include "mature louse," "developed louse," or simply "adult louse." It's important to recognize that while these terms may be interchangeable, they are all referring to the same pest that lives on the human scalp and feeds on blood for sustenance.