A holofilter is a device used to hide a person's real identity from visual contact during communications. The holofilter takes the image from one end and changes it to a new appropriate setting so that when the person moves the holographic image moves the same way.
The USS Defiant was equipped with one of these. In 2371, to enter Cardassian space, Captain Benjamin Sisko used one to make himself appear to be a Kobheerian Captain to Cardassian patrols. (DS9: "Second Skin")
Gul Dukat's Klingon bird-of-prey was equipped with one of these. In 2373 when he transported Worf, Sisko, O'Brien, and Odo to Ty'Gokor, he covered his image with that of a rather imposing Klingon. (DS9: "Apocalypse Rising")