Easy, I'm Welsh/Irish so by heritage I'm a Celt which fits nicely with Celtics.
I imagine most know this but the word Celt (Celts) applies to people whereas Celtic applies to everything else (language, history, dress etc. etc. etc.). You can't have a Celtic person, they're a Celt but you can't have Celt art for example, it's Celtic art. I'm sure you know this already but Celt and Celtic when used about people and culture is pronounced with an initial hard K not a soft S, so phonetically I'm a Kelt and the art etc. is Keltik. Seltik/s and Selt/s refers solely to sporting organisations, most familiar to Europeans in the Glasgow Celtic example and Americans in the Boston Celtics one.
For those interested, the flag below is known as a Celtic (Keltik) flag and brings together the six flags of the principal Celtic nations (starting top left and going clockwise; Brittany, Isle of Man, Scotland, Wales, Cornwall and Ireland). You sometimes see these flags with a seventh or more flag added, most commonly Galicia, which represents places with a strong Celtic heritage but which no longer have strong direct ties to it (modern Galicians for example are not Celts though they still identify very strongly as having a Celtic past).
Incidentally, Celtic cross flags are almost never actually Celtic. They are symbols which have been adopted by fascist movements which is incredibly sad as the Celtic cross is an important part of Celtic heritage and identity, is to be found frequently in graveyards throughout the six Celtic nations and has no political symbolism whatsoever. In the case of Celtic cross flags, they have been misappropriated and b@stardised and have no connection whatsoever with real Celtic crosses. They can be easily identified and differentiated from legitimate use of the Celtic cross design because the fascist version basically looks like crosshairs whereas real Celtic crosses are things of great beauty and intricacy. Glasgow Celtic used the Celtic cross (the real design) on their centenary shirts in 1988:
An example of a Celtic cross from Wales: