Great Soviet mathematician N.I. Akhiezer mentions in his survey article "Чебышевское направление в теории функций" ("Function Theory According to Chebyshev") that the notation $$T_n(x)=\frac{1}{2^{n-1}} \cos{(n\arccos x)}$$ was first introduced by S. Bernstein.
I think that the first published paper on Chebyshev polynomials by Bernstein was "О наилучшем приближении непрерывных функций посредством полиномов данной степени" which appeared in "Сообщения Харьковского математического общества", series 2, vol. 13 (1912), pp. 49-194. The paper is in Russian as you may guess.
In his paper, Bernstein refers to the Chebyshev polynomials as trigonometric polynomials which probably might explain the letter T in the notatin.
English translation of Akhiezer's survey article is contained in Mathematics of the 19th Century edited by A.N. Kolmogorov.