How is this letter or symbol pronounced mathematically? 1)How is this letter or symbol pronounced mathematically?
$$\overline k$$
2) $'$  is this sign just a symbol of derivative? For example: 
$$k'$$ Do we only understand this as a derivative?
 A: It would depend on what mathematical area you were studying when you saw it probably. Just like x isn't always referring to the $x$-axis or $y$ to the $y$- axis on the real plane it could be any Cartesian product etc. $\circ$ could be used as function composition or other compositions as well as in place of the degree symbol if talking about temperature. The verniculum (bar) in fractions and roots means take everything under it together as one thing.  in repeating decimals it is a short way to show what part repeats.  the tick could mean first derivative, it could also mean the preimage of function k in a function context for example.  math symbols get reused a lot. $\aleph$ could be a variable or with a subscript could be referring to a type of infinity for example. 
A: *

*“Kay bar” (or maybe “bar kay” if $k$ is replaced with something long)

*“Kay prime” could hypothetically be used to notate anything, depending on the author and field. However, $k’$ would generally be accepted as any of the following:


*

*the derivative of a function $k$

*the complement of an event $k$ (in which case it would probably be pronounced “kay complement”)

*a symbol that has been distinguished from an otherwise defined but related $k$ (e.g., spacetime dilation, integration dummy variables)


