Ambient life

Scorpions in Act Two.

Ambient life refers to moving things, sometimes clickable that are not monsters or other elements of the game. Birds in the air, snakes and bugs on the ground, etc. These moving things can't be interacted with in any game sense, but they're often a type of destructible since they do react to your presence, and can usually be destroyed if you step on them or cast destructive spells on top of them.

Duriel's death maggots.

Ambient life can be randomly-occurring, like snakes and scarab beetles in the desert, or it can be part of a particular monster or object animation. A famous example of the later type was seen in Diablo II, when dozens of maggots fell out of Duriel's corpse and began crawling in all directions. These bugs were fun to look at and could be squished, but they had no game effect and were not worth experience or items. Not even to stomp-fetish players.

Diablo III Ambient Life

There are numerous examples of ambient life in Diablo III. Even more than in Diablo II. Rats are seen crawling through dungeons, usually along the edges of rooms. Snakes and giant bugs abound in the desert, and the snakes are especially squashable since their AI causes them to flee directly away from any nearby character. (Which means all you have to do is run in a straight line and you'll squash every snake you come anywhere near.)

Blizzard has issued a few brief comments on the game's ambient life.

September 2010: [1]

There are a lot of squishy critters, more of them and more types, and it’s still just as fun to run them over.

January 2010: [2]

Will there be little critters on the ground that you can splash with your boots? I loved it in D2. --Venator

Yes! And there are a ton more. --Diablo