Since a great sword usually sported a longer, heavy blade - and could require two hands to properly wield - the name came to imply any kind of two-handed sword.
While terms do overlap, many kinds of one handed swords also fall under this category - and experts rarely agree on a clear cut definition.
As a matter of terminology, even 18th century broad swords - the curved blade variety, akin to a thick sabre - could be referred to as great swords.