i shall try to clarify in terms of pictures
Structure

this is your typical tank, a weapon turret on the top and a chassis(the main body), the mode of movement at the bottom. as you can see, it hugs the ground very low.

This is also a tank, although it may lack a turret. it is still very much a ground based combat vehicle.

this, for the sake of this contest, is also a tank. it has a weapon turret on the top and a chassis. as you can see, with both of them, the bulk of the vehicle is at the lower part, making them stable. 4 leg is stable

this is NOT a tank, it is a walker. it is classified as a walker, mainly, because it
walks

This is not a tank. it is fourlegged and stable but it does not hug the ground

this, for the sake of this contest, is a mech. it has an identifiable head, torso and arm structure. It is not a tank
Armour
i like to keep it simple. the Grenade test. if it can be knocked off by a grenade, its not a tank. a bicycle is not a tank, a quad is not a tank, a jeep is not a tank. tank is a hunk of metal that will laugh at things like a grenade and small arms fire. if you want a tank that can shrug off a nuke, go ahead. if you want a light tank with very light armour, that can maybe survive a grenade, go ahead. just dont make a golf buggy with guns
Weapon
as said before, any weapons are allowed