Hi Rakesh,
A Rogue Access Point is defined as any wireless Access Point that are not part of the network. It might be operating on the same or an adjacent frequency, occupying the spectrum, raising the noise level (co-channel or adjacent interference) and may or may not be a security risk (unclassified, friendly or malicious Rogues).
Mostly probably its the nearby external APs(from other vendors or other companies).
Rogue Management in an Unified Wireless Network: https://www.cisco.com/c/en/us/support/docs/wireless/4400-series-wireless-lan-controllers/112045-handling-rogue-cuwn-00.html
Regards
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