Not occurring periodically.
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ape·ri·od·ic .ā-.pir-ē-'äd-ik adj
1) of irregular occurrence
2) not having periodic vibrations: not oscillatory
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ape·ri·od·ic (a″pēr-e-odґik) having no definite period; said of membranes that have no definite periods of vibration of their own, but are free to take up any vibrations imparted to them.Medical dictionary. 2011.