The alternative hypothesis, denoted as H1 (or HA) is a conjecture assumed to be true if Ho (the null hypothesis) is false. In practice, this is the state of nature that the researcher believes to be true.
If there is enough evidence against a given null hypothesis, then the null hypothesis is rejected in favor of the alternative. See the concept "null hypothesis" for an illustration.
Additional information on the null and alternative hypotheses can be found in David Lane's text.
HyperStat Online. Copyright © 1993-2001 David M. Lane.