As you ride the bus to work, one thing you're unlikely to hear is Hittite being uttered by the man sitting next to you, or someone screaming at the bus to slow down in Latin. These, among with a host of other languages, are dead. Kaput. Deceased. As a linguist would define it, no children are learning it as a first language. It is a sad, sad phenomenon when languages die, but we've seen (and read about) enough of them to know just how this occurs. We break it apart into four different types of murder:
Sudden Language Death
You wake up one morning, perfectly content to say to your mother, "Hey, what's for breakfast?" when you realize that your entire race of people has died. This is the most dramatic and startling of all the ways a language can die. Simply put, there is no one left to learn it--from the elders down to the smallest child. Such as the case with Tasmanian and Nicoleño (a Native American Indian language once spoken in California).
Radical Language Death
Similar to the sudden variety, radical language death is remarkably abrupt (in the line of languages, that is. 50 years is pretty damn sudden). In this case, however, the language does not die because all the speakers do, but because of pressure to simply stop using the language. Take, for example, many of the native languages of America. In many cases, in an effort to not be portrayed as "native" and, thus, receive the scorn often given to natives, the users of a language prefer to blend themselves into society and never again use the language they were born with. In some cases, it is political pressure or the threat of genocide that can kill a language in this way. When the last active speaker of the language expires (or chooses to adopt a different language), the words die.
Gradual Language Death
Ah, the most common language-killer: apathy.
No, I kid. Gradual language death is the most common, and it occurs when a minority language simply loses the battle, totally, for dominance in any given area. Take Cornish, for example, in the eighteenth century, and a huge volume of Native American languages on both North and South continents.
Someone reminded me that another possible scenario for gradual language death is having a minority language be absorbed into the majority language. As dialects become less and less prevalent, some words and grammatical structures may merge with the dominant language as the recessive slowly dies.
Bottom-to-Top Language Death
Here we come to "dead" languages that survive, against all odds, in some very specific contexts. Take Latin in its liturgical form, or Greek. Latin, for example, stopped being spoken in homes centuries ago, and now lives on in scholarly texts and a variety of Christian masses only--a context-driven language. Hebrew was, at one time, in this category (though after keeping it in the home for centuries, the language resurfaced in a remarkable coup de théâtre as the recognized national language of Israel after the Second World War).
Language death is a perfectly natural thing, just as our own demises are part of natural evolution. Indo-European, the mother of hundreds of languages, had to die, for example, for English to exist at all. Or German. Or French. Hittite passed away 3,500 years ago, and Tocharian bit the bullet in the first century of the last millennium.
Today, many languages are what we refer to as "endangered." Nearly all Native American languages, such as Comanche, Apache and Cherokee (and other names hijacked by SUV manufacturers) have fewer and fewer native speakers with each passing generation. According to some statistics, fewer than 20% of children born to languages such as these are actually learning them.
Linguists keep the hope alive, however, by attempting to preserve endangered languages by studying their grammars (phonetics, phonology, morphology, syntax, &c) and by recording for posterity the last remaining speakers of the language. More important that that, however, is recording the literature (poetry, books, rituals, &c) in its native language. By examining these bits of whispered words on the wind, we're able to get new insight into the nature of human cognition, recognizing the intellectual achievements of a culture that give new perspective to the human condition.