You know, if we DO make contact with aliens within the next hundred years-ish, enough people are going to give the Vulcan salute to the first alien they see in real life (whether out of quivering excitement, lol memeitude, or sheer awkward, panicked grasping for the one prominent alien greeting nested in the social conscious) that there's a solid chance that it becomes a thing we're known for as humans.
Live Long and Prosper, my friends.
Leonard Nimoy would openly, unironically weep with joy if that happened.
He helped develop the Vulcan salute based on what he saw certain people doing during services at his temple. The splayed fingers represent the Hebrew letter shin ש, and the men (I don’t know if they are rabbis) do it with both hands. It represents a blessing.
In an interview, Nimoy mentioned how delighted he was that people the world over greeted each other with “the Vulcan salute,” because it meant “they were going out and blessing each other.”
I’ve always been an atheist, but I still think it’s a lovely sentiment to greet someone with “May you have a long and bountiful life, and I wish for good things to happen to you.”
It's an excellent greeting.
It's non-threatening. You not only can't hold a weapon in that hand, the position itself is awkward enough to be useless for combat. It's not something that easily switches to a punch or strike.
It doesn't involve touch contact - avoids both cultural barriers to touch and problems of disease or incompatible skin types. (Don't have to shake the hand of the volcano aliens.)
It's clearly deliberate, unlike some kinds of bowing. And it can be mirrored by a wide range of body types, even if they don't have "hands" or "fingers" - an octopus-being that holds up a tentacle in response would be recognized.
And the sentiment it conveys (the Vulcan version; I have no idea about the original Jewish meaning) is likewise pretty universally acceptable.
It’s the Priestly Blessing (although in some Reform synagogues the rabbi will do it rather than calling up all congregants of priestly lineage to do it--i.e. everyone whose last name is Cohen or Katz or Kaplan or whatever is supposed to go up and collectively bless the community together).
The words match the oldest excerpt of the Bible found by archeologists, on amulets in grave goods dating from before the big redaction project that actually produced the Bible in its written forms. (NB: Jewish amulets usually take the form of written text; this is even more true in the modern era.) There is a real sense in which this blessing predates Jewish religion as we usually understand it.
The translation is “May HaShem bless and keep you, may HaShem’s face shine on you and show you favor, may HaShem lift his face to you and give you peace.”
In the Jewish religious context, “may HaShem bless and keep you” means, like, may God decide that you’re going to keep living a while longer. So the benediction literally means Live Long and Prosper in Peace.
