"A variable is a node serves as a template or a query for ..."
=>
"A variable node serves as a template or a query for ..."
"A variable is a node serves as a template or a query for ..."
=>
"A variable node serves as a template or a query for ..."
Made it:
A <firstterm>variable</firstterm> is a node that serves as a template or a query for other XDI graphs.