Here's the history:

* Lead was added to gasoline for decades. It was a "knock suppressor", helping to prevent engine knocking (a harmful detonation).

* Lead is a serious health hazard. It pollutes the air and can cause brain damage, especially in children.

* Environmental regulations led to the phasing out of leaded gasoline. This started in the 1970s and continued into the 1990s, depending on the country.

Today, most cars are designed to run on unleaded gasoline, which uses other additives to prevent knocking.

So, "unleaded" is simply a description of the fuel, emphasizing the absence of lead.