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Sorry for such a newbie question. I searched and read multiples sources about supernova but I'm not completely sure if what is called "supernova" is the explosion, the resulting celestial body or the event as whole (explosion plus resulting celestial body).
Would it be considered as incorrect if for some reason the explosion of a star is called as "supernova"?
A supernova is the explosion, visible on Earth as a apparently new star that appears to brighten over a few days, and then fade. There have been no visible supernovae in our galaxy for several hundred years, and we are well overdue another one, though they are quite often visible in other galaxies.
After the supernova there will be a remnant, a small nebula of hot gas that expands, eventually taking the form of a ring. In the centre of the remnant there may be neutron star or a black hole. A spinning neutron star may be a pulsar.
Supernovae that occur when a old, giant star explodes at the end of its life can produce a neutron star or black hole. But some supernovae form when a white dwarf accumulates matter from another star onto its surface, and then explodes, in which case the star is completely destroyed.
The term "nova" was previously used for all stellar explosions. However the term "nova" is nowadays limited to smaller explosions, caused by the fusion of hydrogen on white dwarf (but no sufficient build up of hydrogen to cause the white dwarf to be destroyed)
Is it incorrect to call the explosion “supernova”?
Yes and no.
Better said, the explosion is the very first part of a supernova. While the explosion lasts for but a few seconds to a few hundreds of seconds, a supernova can last for hundreds of days. What we see visibly as a supernova are the after effects of that explosion. The explosion can create lots and lots of stuff moving at very high velocities, and lots and lots of highly radioactive nuclei.
If the star had outgassed material prior to the explosion, the highly kinetic material produced by the explosion runs into that previously outgassed material and makes it glow. This takes some time to cool down. The radioactive material produced during the short course of the explosion proper takes time to decay to stable elements. This radioactive decay eventually produces gamma rays, some of which is absorbed by the nearby material, heating it, thereby eventually producing thermal radiation.
For example, a type Ia supernova produces a large amount of nickel-56. This decays to cobalt-56 with a half-life of about 6 days, which in decays to iron-56 with a half-life of about 77 days. The heating that results from these decays is what we see, and because it is so predictable, this is makes type Ia supernovae a fantastic standard candle.