Per the AHS Dictionary:
Dormant - the term "dormant" refers to daylilies that lose their foliage completely before or shortly after frost and over-winter with pointed foliage buds, usually just beneath the soil surface. Dormants will resume growth in spring.
Evergreen - daylilies that retain their foliage throughout the year. In cold winter climates, evergreen daylilies over-winter as a mound of frozen pale green foliage. Evergreens may resume growth during a mid-winter thaw in mild climates.
The semi-evergreen definition is pretty vauge, but I think it refers to plants that may show some dormancy, but not completely die back in winter.
The tricky part is that many plants may behave differently in different climates. Some evergreen daylilies in my southern garden might go dormant if transplanted in a northern zone, and vice versa. Many dormant plants have been hybridized in colder climates, and are sometimes considered more cold hardy, but in fact, plant habit is not necessarily an indication of hardiness. There are hardy and tender plants of all foliage types.
I'm not sure if dormancy is dominant or not. Daylily genetics is a very complicated subject, so I will defer to those who are more experienced on that one.
Hope this helps!
