Demisemiquaver(s)- the s makes it a 15 letter word. I hope this was the one you were looking for!
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IroquoisIndians
Defencelessness
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My name is Radia and I'm 15 (years old). Note - In French, the word for word translation is: I call myself Radia and I have 15 years
"Electroencephalograph" is a 15 letter word that has the letter "e" five times and no other vowels. It refers to the measurement of electrical activity in the brain.
Demisemiquavers! A demisemiquaver is half the duration of a semiquaver, quarter of the duration of a quaver, an eighth of the duration of a crotchet etc. It looks similar to a quaver but has 3 lines coming out at the top instead of 1.
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Punctuation mark is another word for parentheses or brackets.