inter partes reexamination

In United States patent law, a reexamination is a process whereby a third party or inventor can have a patent reexamined by a patent examiner to verify that the subject matter it claims is patentable. In order to have a patent reexamined, the party of interest must submit prior art that raises a "substantial new question of patentability".
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-- see reexamination proceeding 

Inter partes reexamination practice primarily differs from  ex parte practice in that the third party requestor may file written comments addressing issues raised by the patent owner in a response to Office action.

-- see 35 USC 311 and 37 CFR 1.902-1.997 and MPEP 2600 for more

Context: Patent