The defining characteristic of plastic would seem to be the definition of the word from which they are named. That is, that plastics are plastic. It means they are easily shaped, molded, formed, or extruded.

That's what they told me in school, too.

But then I got out in the real world, and noticed plastics just falling apart all around -- including stuff that is not intended to fail and which is otherwise still within its useful life.

Like: One year, I bought some used pickle buckets from a local burger joint to use as planters. Within 6 months, they were falling apart: It was easy to break them apart in chunks with my bare hands.

Or the plastics used for cars: They often eventually turn brittle and fall apart, whether interior or exterior. Plastic lenses on USDM cars turn foggy and useless; some types of wire insulation disintegrate. (If we want to talk about environmental cost, can we also talk about the impact of building a new car?)

In some areas, we once used polybutylene water pipes. These tended to fail and damage homes. There was even a billion-dollar lawsuit about it in the 1990s. It was not good.

Meanwhile, a red Solo cup or a plastic drinking straw, once landfilled, will be there a very long time -- but eventually, they will also decompose.

And the UHMW cutting boards I use in my kitchen will probably outlive my grandchildren's grandchildren before they start falling apart on their own accord.

Plastic isn't always forever, even though some people seem fond of saying that it is. Plastic isn't necessarily cheap, either, even though "cheap plastic" is a common expression -- some plastics are very expensive and resoundingly durable (and there's only partial overlap of these two qualities on a Venn diagram).

The truth is somewhere in the middle, but is rather nuanced and variable and difficult to pin down in absolutes.

But plastic (as a noun) does, broadly speaking, have the material property of being plastic (as an adjective).

There are a lot of plastics that are disposable. The first plastics over 100 years ago were made from milk (I suspect the Romans knew the process - it wasn't industrially useful though). Many decomposable plastics have been made over the year. The PLA commonly used in 3d printers is decomposable (in the right environment).

Oil based plastics are generally a lot cheaper than the above though, and they are typically not decomposable. Depending on your use this can be good or bad (I don't want my plastic plumbing pipes to decompose, but other plastics are used up and I want them to decompose)

> Plastic's defining characteristic is that it is not decomposable

Behold, a plastic! holds up a rock

What? Plastic's defining characteristic is plasticity -- how moldable, shapeable, extrudeable, pressable, etc it is.

Also, one of the most widely used plastics is PLA, polylactic acid. Which is made from lactic acid from sugarcane, beets, or cassava, and is biodegradable.