Meaning of mathematical operator that consists of square brackets with a plus sign as a subscript

I was reading a paper on tomographic reconstruction, and I found an operator that is not explained:

$[expression]_+$

The operator was used to compute a surrogate for the log-likelihood cost function. I do not know what that operator means. I've seen brackets without that plus sign before that were used to represent the rounding operation.

Thanks!

Edit: I've been looking at http://www.latexsearch.com and I found some results where $[x]_+:=\text{max}\{0,x\}$, and I think this agrees with how the paper uses it.

Looking at other papers I found through Latex search, I found that the bracket operator is defined as: $[x]_+=\max\{0,x\}$