1 : a usually foliated ornament forming an upper extremity especially in Gothic architecture
2 : a crowning ornament or detail (as a decorative knob)
This is a pretty broad definition, so I found two examples of finials to help us understand this thing better. One is stand alone, and the other serves as a resting place for a candle or some other object. They can also be used as bases in lamps!
I like this new word, and I like what it is! Enjoy!