"Cart Subtotal"

For anyone seeking the answer, the code to change is in plugins/woocommerce/includes/class-wc-order.php

If it’s just for the PDF Invoice, then you can also change this with a filter in functions.php:

add_filter( 'wpo_wcpdf_woocommerce_totals', 'wcpdf_subtotal_label' );

function wcpdf_subtotal_label($totals) {
	foreach ( $totals as $key => $total ) {
		if ( $key == 'cart_subtotal' ) {
			$totals[$key]['label'] = 'Subtotal';
		}
	}

	return $totals;
}

IIRC, woocommerce has a similar filter (this saves you the hassle of editing template files).

Thanks as always Ewout!