Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. A quantum number equal to twice the average electric charge of a particle multiplet or, equivalently, to the sum of the strangeness and the baryon number.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. A quantum characteristic of a group of subatomic particles governed by the strong force that is related to strangeness and is represented by a number equal to twice the average value of the electric charge of the group.
Etymologies
from Wiktionary, Creative Commons Attribution/ShareAlike License
Examples

GellMann's proposed the new rule: Elementary particles can be transformed in others by the strong and the electromagnetic interactions only if the total hypercharge is conserved.

You might think that you could just have a single SU (2) L or U (1) hypercharge gauge boson connect that loop to the Standard Model fermion ψ, but that vanishes by gauge invariance; you need two gauge bosons, and thus two loops.

In the original version of this post (and in the original version of our paper), I claimed that you would need a three loop diagram in the case where the dark matter had zero hypercharge (so you had to use SU (2) L gauge bosons, which couple only to the lefthanded fermions).

The only thing about this which is slightly complicated is the U (1) hypercharge assignments.

This one, the creation of Sheldon Glashow, Abdus Salam, and Stephen Weinberg, combines SU (2) xU (1) with the thenlittleknown Higgs mechanism to produce a theory that conserves “weak hypercharge”, and accounts for both the weak interactions and the photon (electromagnetism).

He introduced a new fundamental characteristic of a multiplet called its hypercharge.

It should be remarked that GellMann initially used instead of the hypercharge a closely related number called the strangeness.

However, we only include the fundamental coupling, including the hypercharge U (1) group.

U (1) generator "Y" (hypercharge) and the third, zrotation generator of the "isospin" SU (2):

At extremely high energies, well behind the right end of the graphs above, the hypercharge
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