Hypernova is an astronomical term whose meaning has been shifting. In the late 1990s, it was used to mean an explosion with energy of over 100 supernovae (1046 Joules). Such explosions were proposed to explain the exceptional brightnesses of gamma ray bursts. More recently, hypernova has been used to refer to an exceptionally large star that collapses at the end of its lifespan (this is known as a collapsar).
No hypernovae of the earlier type have been definitively observed, and the explanation has fallen from favor as it has become clear that gamma ray bursts are beamed into very narrow opening angles of less than 1% of a sphere, so that ordinary supernovae can power them.
In the collapsar model, the core of the star collapses directly into a black hole and two extremely energetic jets of plasma are emitted from its rotational poles at nearly the speed of light. These jets emit intense gamma rays, and are a candidate explanation for gamma ray bursts. In recent years a great deal of observational data on gamma ray bursts significantly increased our understanding of these events, and made clear that the collapsar model produces explosions that differ only in detail from more or less ordinary supernovae. Nevertheless, they continue to sometimes be referred to in the literature as hypernovae.
Since stars sufficiently large to collapse directly into a black hole are quite rare, hypernovae would likewise be rare, if they indeed occur. It has been estimated that a hypernova would occur in our galaxy every 200 million years.