Now, the VOA Learning English program, Words and Their Stories.

You hear the word sustainable often in the news and in conversation. Its meaning has changed in recent years.

Sustainable is an adjective, so it usually appears before a noun. Before the 1960s and 1970s, the word usually meant ‘bearable' or ‘defensible.' Now, it also means "able to last or continue for a long time."

The first record of the modern use of the word referred to sustainable growth. This is an approach to business growth that can continue without creating too many problems.

In the 1970s, as awareness of the environment grew, people became more concerned with the effects humans have on the earth. We need to eat, of course, but we want our food to come from sustainable agriculture - farming methods that do not completely use up or destroy natural resources.

Experts in sustainable agriculture talk about a related concept, biodiversity. We know that diversity refers to the variety of people or cultures in a group. Biodiversity is a measure of how many varieties of living things such as plants or animals are in an environment. "The more the merrier" is the thinking behind this term. Growing only one kind of food in an area is not sustainable because there is no biodiversity. Have you heard of the Irish Potato Famine? In the Nineteenth Century, when the potato crop failed, there was no other crop to eat.

Organizations that want to show their concern about the environment may say they are green, which means they are trying to protect the natural world. Recycling, reusing, and reducing waste are some of the ways we show how green we are.

A sustainable source of energy does not go away. For example, solar energy is sustainable because the sun will always be shining. A related word is renewable. A renewable source of energy is one that has an unlimited supply. Fossil fuels, on the other hand, are not renewable. Energy based on oil is not renewable because there is a limited supply of it.

Smart growth also is related to being sustainable. In many cities today, young people are moving to places where they can walk or take public transportation instead of driving a car. Smart growth results in these kinds of neighborhoods. They are walkable urban centers meant to avoid sprawl, the expansion of cities to new areas. This kind of growth can continue a long time, because it uses less land and resources.

It is safe to say, you will hear this word very often in the future.

I'm Jill Robbins.

Dr. Jill Robbins wrote this story for Learning English. Mario Ritter was the editor.

Now it's your turn. Use the word sustainable in a sentence in the comments or give your opinion on this topic. What is the thinking in your country about sustainable development?