Lord

Lord is a generic term for the commanding leader individual in several armies.

Dark Eldar Lord

Can be either an Archon or Dracon

Can be either an Archite or Dracite

Chaos Lords are the highest ranking warrior, often having led armies for hundreds or thousands of years.

Necron Lords are the leaders of Necron forces who partially retain their free will an are able to direct other Necrons in more imaginative ways.

Lord General is the highest rank possible within the Imperial Guard. Within the Imperial Navy, the highest rank is Lord High Admiral, with a subordinate rank of Lord Admiral.