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

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adj. Of or relating to the ordinal number matching the number 0 in a series: the zeroth power of 10.

adj. In the first position in a sequence whose elements are numbered starting at zero; the ordinal number corresponding to zero.

adj. Corresponding to a position preceding the first.

adj. preceding even the first

"The previously discussed omnidirectional patterns are also called zeroth-order patterns and the other patterns mentioned previously (dipole and cardioid) are called first-order patterns."

ordinal   

0th    null