The choice between two mutually exclusive possibilities, the situation presenting such a choice, either of these posssibilities. 

One of a number of things from which one must be chosen. 

This word is an accomplice to a myth in the expression "alternative energy technologies."  I wrote a whole book of essays (A Certain Bicyclist) to dispel it.  Here's a summary.

• Petroleum is the most precious natural resource.
• Petroleum is the cheapest form of energy based on its intrinsic costs.
• Petroleum is non-replenishable.

Clear thinking begins when you make explicit those assumptions you are not aware you are making.  My apocalyptic view derives from a strict definition of the word "alternative," which gives emphasis to the hidden assumption of interchangeability.

Policy-makers and the public-at-large do not make that assumption explicit.  When they speak of alternative energy technologies, they typically assume, without saying so, that modern science will inevitably create an economical substance which replaces petroleum and allows life as we know it to continue unchanged.

More like a miracle is required.  Careful consideration of each so-called "alternative" does not support optimistic assumptions, much less mindless expectations.  Five decades studying this matter has left me no alternative but to conclude that life on Earth will be radically different in the Post-Petroleum Age.

Epilog: 

Some 14 years after publishing this essay, I had a conversation with a certain French Woman about a party she had attended at a home in Berkeley. Without irony, she described her hosts as “very alternative.”  I was instantly charmed at the exceptions to my personal proscriptions -- two at the same time.   Thus, I am henceforth prepared to tolerate the use of alternative if and only if that dreaded word is immediately preceded by another, very.