x′ = x + t
"Ex (?) equals ex plus tee".
In Russian it is called "штрих" (shtrikch).
The single tick following a variable is often (but not always) used to represent a derivative and (in the United States) is always pronounced "prime." In your example, "Ex prime = ex plus tee."
f(x) = x² <--- "Eff of ex equals ex squared."
f′(x) = 2 x <---- "Eff prime of ex equals two ex."
f′′(x) = 2 <---- "Eff double prime of ex equals two."
In non-mathematical contexts it is called a single quote (or a "tick"). This wikipedia entry differentiates between the prime symbol and the single quote. As they also note, using a single quote (') as a stand-in for prime (′) is not uncommon. Thanks Vi for the link.
I have learned from other respondents that in the UK, Canada and Australia, it is pronounced prime unless it signals a derivative, in which case it can be pronounced dash.
In case you run into these two:
x̅ is pronounced "ex bar"
x̂ is pronounced "ex hat"
In my experience, in the specific case where it indicates a derivative, it is pronounced "dash". Odd, I know, as it does not look anything like a dash "-".
In all other circumstances, "prime".
Found this nice general reference on mathematical and scientific symbols pronunciation that may be useful.