What is Sargability?

I planned to attend Rob Farley's 24HOP session but could not make it. I didn't know what Sargability meant at the time and thought I will google it.

And here is what I have found out thanks to the internet.

Here is a blog post which gives the history of how the word came into existence back in 2005 where Brad McGhee explains that the sargability is a real word but it is used a lot in the SQL Server world.

So what exactly does Sargable mean?
SARG is a contraction for Search ARGument ABLE as mentioned in Rob Farley's article.

I am looking forward for the recording of this 24HOP session. Meanwhile you can have a look at the posting by Rob Farley in his blog where he gives links to the resources he has mentioned in his presentation.