This word is acceptable for play in the US & UK dictionaries that are being used in the following games:

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adj. occurring every six months or during every period of six months

"The pavid matron within the one vehicle (speeding to the Bank for her semestrial pittance) shrieked and trembled; the angry Dives hastening to his office (to add another thousand to his heap,) thrust his head over the blazoned panels, and displayed an eloquence of objurgation which his very Menials could not equal; the dauntless street urchins, as they gayly threaded the"

periodical    periodic   

semestral