A person of the same generaton as you that shares a common set of great-grandparents, just as a cousin is someone of the same generaton as you that shares a common set of grandparents. Not the same thing as a cousin once removed.

great-grandparents

/                           \

grandparent               great aunt/uncle

/          \                                     \                

         parent    aunt/uncle       second cousin once removed

|                         |                                         |               

YOU              cousin                            second cousin