Dear bridget96: please forgive the goofy way my computer mistranslates some of the symbols used by my CD dictionary.
And the fact that I don't have much to add to what it says.
So, by the third definition a cognomen is any name, often a nickname.

Incidentally, I think the "handles" used by board members would be closer to "alias" than to "cognomen".

Dcog[no[men 7k9g nb4m!n8
n.,
pl. 3no4mens or 3nom$i[na 73n9m4i n!8 5L < co3, with + nomen, NAME: sp. infl. by assoc. with *gnomen < Gr gnbma, mark, token: akin to L gnoscere, KNOW6
1 the third or family name of an ancient Roman (Ex.: Marcus Tullius Cicero)
2 any family name; surname; last name
3 any name; esp., a nickname
cog[nom$i[nal 73n9m4i n!l8
adj.