What is the programming term for the command invoking '\' in latex?

Not any character or action is available from a keyboard input as such an escaping to the 'TeX' system is necessary to typeset certain symbols or trigger actions -- in order to distinguish the text "foo" from a an action (call it macro etc.) named "foo" as well a certain method is necessary.

D.E. Knuth designed \ to be \ the “escape char” and is used to “escape” to commands, i.e. generate certain actions either in full macros, control words like \foo or as a control symbol like \# etc. to display special characters that itself have a special meaning (a different catcode, such as # or $ etc.).

The escape character has been assigned the catcode 0.

D.E. Knuth states in The TeXBook (see chapter 3, 3rd. paragraph, but see chapter 7 also):

TeX allows any character to be used for escapes, but the 'backslash' character '\' is usually adopted for this purpose, since backslashes are reasonably convenient to type and they are rarely needed in ordinary text. Things work out best when different TeX users do things consistently, so we shall escape via backslashes in all examples of the this manual.

Basically, any character could serve as escape character, i.e. having the meaning of “catcode 0”, see the example below -- but I don't recommend it. ;-)

\documentclass{article}

\begin{document}
\catcode`?=0

$?frac{1}{2}$

?section{Foo}

\end{document}

Even this is possible -- but don't do it (at least, I don't recommend this!)

\catcode`?=0

?documentclass{article}


?begin{document}

$?frac{1}{2}$

?section{Foo}

?end{document}

Related question: Is there a way to eliminate the “\” in frequently used LaTeX commands?

Here's a short overview of characters with must be escaped with \:

\ { } $ & # ^ _ % ~

all require a \ to be typeset directly, i.e. use

\\ \{ \} \$ \& \# \^ \_ \% \~

The term for this phenomenon is stropping