There is no standard symbol for this in LaTex. This symbol is often defined as:

```
\def\ci{\perp\!\!\!\perp}
```

You can see it in use on the wikipedia page for Conditional Independence.

If there's such a thing as an 'offical' definition, there's this from the Comprehensive LaTex Symbol List page 106:

Donald Arseneau posted the following **\mathpalette**-based definition of a probabilistic-independence symbol *⊥⊥* to comp.text.tex in June 2000:

```
\newcommand\independent{\protect\mathpalette{\protect\independenT}{\perp}}
\def\independenT#1#2{\mathrel{\rlap{$#1#2$}\mkern2mu{#1#2}}}
```

The **\independent** macro uses **\mathpalette** to pass the **\independenT** helper macro both the current math style and the **\perp** symbol. **\independenT** typesets **\perp** in the current math style, moves two math units to
the right, and finally typesets a second—overlapping—copy of **\perp**, again in the current math style. **\rlap**, which enables text overlap, is described later on this page.

Frankly, I like the first version better. It might not always render properly in all math fonts, but I can remember it when I need it. Sorry, I don't know how to do this in the equation editor.