What is silhouette?

Solution 1:

man silhouette reports (rather unhelpfully):

silhouette -- generate local topics on the device.

The daemon is launched by /System/Library/LaunchAgents/com.apple.coreparsec.silhouette.plist which is part of CoreParsec, the private framework that is part of Siri's backend.

strings /usr/libexec/silhouette yields a lot of method names related to machine learning: scoring bins, bloom filters, normalization routines, false positives, etc.

Absent further documentation from Apple, it's reasonable to conclude that this daemon is part of Siri's infrastructure. "Local topics" has a specific meaning in machine learning (usually in context of clustering algorithms), though I'm not sure if it's being used in that context or not.

It is for sure not malware, if that's what you're concerned about.