invalid attribute 'target'

Hi,

I was validating: http://www.graszodenland.nl/graszoden-kopen
and i received this error.
 Line 358, Column 66: there is no attribute "target"…3 <a href="http://www.conceptnoord.nl" target="_BLANK">Concept Noord</a></span>What is wrong with the target='_BLANK'?Can anyone help me? Thanks in advanceEvelien