I thought it might be interesting to repost two OED entries Interesting because it records a few sentences where the word was first used, as well as where it was found and what year:
Buddha The title given by the adherents of one of the great Asiatic religions, thence called Buddhism, to the founder of their faith, Sakyamuni, Gautama, or Siddartha, who flourished in Northern India in the 5th century B.C. Sakyamuni is regarded as only the latest of a series of Buddhas or infallible religious teachers, which is hereafter to be continued indefinitely. When applied to Sakyamuni, Buddha is in English use treated as a proper name, and even when used in a general sense, it is always written with a capital B.
1681 R. KNOX Hist. Ceylon 18 The Buddou, a great god among them. 1784 SIR W. CHAMBERS in Asiat. Res. (1799) I. 163 The Siamese have two orders of priests, and so have the worshippers of Buddou. 1803 MAHONY in Asiat. Res. VII. 32 This last Bhooddha will be born of a Braminee woman. 1844 H. H. WILSON Brit. India II. 87 With the acquiescence of..the priests of Buddha. Ibid. III. 50 Relics of the four last Buddhas. 1853 WAYLAND Mem. Judson App. II. 410 A Buddh is a being who by virtue of..certain austerities becomes the object of supreme adoration. 1858 MAX MÜLLER Chips (1880) I. ii. 51 The first subjective system of faith in India, the religion of Buddha.
attrib. 1784 SIR W. CHAMBERS in Asiat. Res. (1799) I. 163 Knox says of the Buddou Priests, etc. 1801 JOINVILLE in Asiat. Res. VII. 421 Some prince on the continent, professing the Boudhou religion.
Hence Buddhahood, the condition of a Buddha; Buddhaship, the office of a Buddha.
1837 G. TURNOUR Maháwanso I. xxviii, Prince Siddhattho attained Buddhohood, in the character of Gotamo Buddho. 1878 DODS Moham., Buddha & C. iii. 147 Such then was the process by which Siddartha painfully won his way to Buddhahood. 1882 SCHAFF Encycl. Rel. Knowl. I. 333 Gautama's Buddhaship was for five thousand years.
Buddhism The religious system founded by Buddha.
1801 JOINVILLE in Asiat. Res. VII. 400 If Boudhism could not have established itself among the Brahmins, etc. 1816 Asiatic Jrnl. I. 19 The name and peculiarities of Buddhism have a good deal fixed my attention. 1870 F. HALL in Wilson Vishnu Purána V. 376 The Hindus, with their hatred of Buddhism and everything therewith cognate.