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

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ad. To a degree that is negligible; so small or slight that it may be ignored.

In a quantity or to a degree which may be disregarded.

"Phenytoin has been found to improve atrioventricular conduction and negligibly affects intraventricular conduction."

negligent    negligible    neglectful    neglect    negligibility   

insignificantly