It's called a fist bump .
It's also known as pound or dap .
ell.stackexchange.com/users/21978/shin
I agree with the dominant answer, but to be a tad more pedantic:
We call this gesture, as a noun:
They close the deal with a handshake .
The close the deal with a fist-bump .
As a verb: Whenever we meet, we shake hands .
Whenever we meet, we bump fists .
QED.
ps:
A fist-bump is demonstrably cleaner than a handshake, epidemiologically .
ell.stackexchange.com/users/32841/Alpinwolf
Aside from the more formal "fist bump", a more slang term is brofist .
this term is quite popular online, to the point that major Youtube celebrity Pewdiepie made it part of the title of his videogame.
ell.stackexchange.com/users/8368/Nzall
If your business is with a male fan of My Little Pony: Friendship is Magic, then it is customary to seal the deal with a "brohoof" -
again, this is just for brony business.
Urban Dictionary: brohoof:
Friendly gesture between one brony and another in the form of connecting knuckles together (or hoofs).
ell.stackexchange.com/users/32878/Brohoof
They closed the deal with a handshake.
The closed the deal with (a) fistbump .
Whenever we meet, we shake hands.
Whenever we meet, we fistbump .
This is a really informal gesture, so I don't think it could be used to close deals.
Since it's informal, I don't think there's any proper conjugation for it either, but it would probably follow the conjugation of the word bump .
ell.stackexchange.com/users/32918/cathygomez
We all know shaking hands or a handshake. There, we 'shake hands'.  There's one more gesture I do. I gently  punch  other's fist.   Something like this -   What do we call this gesture as  a noun ?    They close the deal with  a handshake  The close the deal with  (a?)  ____________   Also, as  a verb?   Whenever we meet, we  shake hands  Whenever we meet, we ___________   If there's no single word, a couple would do.
What do we call the gesture of hitting each other's fists gently?
ell.stackexchange.com
<word-request><phrase-request>
ell.stackexchange.com/questions/87416
ell.stackexchange.com/users/3187/Maulik V
This is known as a fist bump, brofist or brohoof. This is an informal gesture known as a fist bump. It can also be referred to as a brofist or brohoof (male My Little Pony fans).
This is an informal gesture known as a fist bump. It can also be referred to as a brofist or brohoof (male My Little Pony fans).