x′ = x + t
"Ex (?) equals ex plus tee".
In Russian it is called "штрих" (shtrikch).
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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.
In High School and Junior High in Japan, we read
a′ as "a dash."
However, in universities, it is occasionally read as "a prime" due to American influence.
The Oxford English Dictionary (1969) states that it is "usually read as 'a dash' in the explanation of the word 'prime'."