Move definition from one context to Global`

If you really want this, you can do e.g. the following:

Block[{$ContextPath = {"System`", "DEF`"}}, 
  Scan[
    (Context[#] = "Global`") &, 
    Names["DEF`*"]
  ]
] 

Here, Block was used just to make sure that Names will return short symbol string names, rather than fully-qualified ones.

This will move all symbols from DEF` to Global`, which automatically make DEF` context empty, and it will be then automatically removed from a list of available contexts, as per Contexts[].

On my system, Mathematica 10.1 under Windows, I got an error using Leonid's code:

Context::cxdup: Cannot set Context[DEF`x] to Global`, since a symbol already exists with name x and context Global`. >>

I seem to need something like this to avoid it:

Block[{$ContextPath = {"System`", "DEF`"}}, 
 Scan[(Quiet[Remove @@ {"Global`" <> StringTrim[#, __ ~~ "`"]}]; 
    Context[#] = "Global`") &, Names["DEF`*"]]]