Complete shitpost, don't take it seriously. We were discussing #define int long long and I suggested defining it to one letter if you want to save on typing so desperately. jeroenodb said that those are reserved for variables, which prompted me to think, "Is there a letter I don't standardly use for some particular purpose?" Then I compiled the following list, and then I thought, "Well, I already wrote it, let's put it on cf", so here we are.

a - array, but also all sorts of stuff
b - second array
c - third array / count
d - delta / divisor
e - event / edge
f - flag
g - graph / second flag
h - third iterator / third flag / height
i - iterator
j - second iterator
k - third input size / some parameter (i know, those are all parameters, but for some reason k is parameter)
l - left
m - number of edges / second input size
n - input size
o - ???
p - permutation / position
q - number of queries / query / second permutation / second position
r - right
s - string
t - tests / time / temp
u - second vertex
v - vertex
w - weight / value (because v is vertex???)
x - variable in math sense / x coordinate
y - y coordinate
z - use when need some letter that won't be used / z coordinate