[angkor]

"...the acquisition of the contents of communications transmitted by means of communications used exclusively between or among foreign powers,..."

All you needed to do was look at 50 USC 1801 (a)(1), which is specifically referenced above:

"As used in this subchapter: (a) “Foreign power” means.... ( 4) a group engaged in international terrorism or activities in preparation therefor;"

[OXENinFLA]

But 1802 doesn't cite (4), just 1,2,&3, under 1801.