I understand that reads to /dev/random may block, while reading /dev/urandom is guaranteed not to block.
Where does the letter u come into this? What does it signify?
Userspace? Unblocking? Micro?
Update:
Based on the initial wording of the question, there has been some debate over the usefulness of /dev/random vs /dev/urandom. The link Myths about /dev/urandom has been posted three times below, and is summarised in this answer to the question When to use /dev/random vs /dev/urandom.
Unlimited.
In Linux, comparing the kernel functions named random_read and random_read_unlimited indicates that the etymology of the letter u in urandom isunlimited.
This is confirmed by line 114:
The /dev/urandom device does not have this limit [...]
Update:
Regarding which came first for Linux, /dev/random or /dev/urandom, @Stéphane Chazelas gave the post with the original patch and @StephenKitt showed they were both introduced simultaneously.
/dev/random named /dev/lrandom? :) random_read :) random v. lrandom doesn't hold all that well since both random and urandom were introduced simultaneously in the Linux kernel. It depends on which "Unix" system you are talking about.
On FreeBSD, /dev/urandom and /dev/random are the same device. The letter u is now a historical legacy that exists for backward compatibility. At startup, they block until enough entropy has been gather and then never block again. See Myths about urandom for details.
In the modern Linux world (starting with kernel 4.8), both devices pull from the same CSPRNG, so the only difference is that some people speculate at the existence of an attack. This attack is like FTL [Faster than Light] travel. Easy to speculate about, rather hard to actually design.
TLDR is just use /dev/urandom.
FreeBSD does the right thing: they don't have the distinction between /dev/random and /dev/urandom, both are the same device. At startup /dev/random blocks once until enough starting entropy has been gathered. Then it won't block ever again. Uniform. Random numbers uniformly distributed between 0 and 1. A flat distribution... as opposed to a peaked distribution like Poisson, or Normal/Gaussian.
/dev/random and /dev/urandom both give the same distribution, and in any real sense it's not uniformly distributed between 0 and 1.
/dev/randomwhich is blocking./dev/randomwas somehow better than/dev/urandomis now long obsolete and for the vast majority of use cases,/dev/urandomis now preferred.