A mannerism was a quirk particular to one or one's species.
In, Spock followed James T. Kirk's line of reasoning regarding the Yangs' development, suggesting they might have developed stoic mannerisms, among other things, much as Humans would have if they had been driven out of their cities into the hills and deserts. (TOS: "The Omega Glory")
In 2269, Anne Nored compared her fiancée Carter Winston to the Vendorian imitating him, noting their similar mannerisms. The Vendorian explained that it was a side effect of remaining in Winston's form. (TAS: "The Survivor")