Zombie: undead corpse.
-=-No higher intelligence (the reasoning power of a venus fly trap),
-=-Aggressive (possibly heat seeking),
-=-Animated as opposed to Alive- dead meat that somehow is being puppeteered
I see no reason why zombies might not start off as "just dead" that walk- in others words, no decay to speak of. In a matter of hours after death the necrotic changes begin though- stench, milky eyes, discoloration. From there on in it's a battle between the physics and chemistry of their biological state and whatever force is animating them. Likewise there will be a struggle between scavenging plants, animals and fungi and the zombie's undead flesh.