of course, is the great Hercules, as the Romans called him, and here we have the most famous and popular of all Greek heroes. (With the Romans, he was more a god of physical strength, an image that he still popularly bears today, as a kind of classical Tarzan or 'Superman'.)
His name has been the subject of much argument and speculation, with some sources saying flatly 'origin unknown' and others boldly asserting that it means 'Hera's glory'. This latter is indeed the widely held derivation, the interpretation being that he was 'one to whom Hera gave glory'. There is one serious drawback to this, however, as hardly any Greek god has a name that is compounded of that of another god or goddess.
Since we are told in the tales about him (by Pindar and others) that his original name was Alcides, we may perhaps conclude that Alcides was his divine name and Heracles was his human name - in other words the 'glory of Hera' was the gift of a son to his parents.
This is to suggest, in turn, that there may even have been a real man who became famous for his exploits, and that Heracles developed out of this folk hero. Even here, though, there are difficulties, since Hera actually persecuted Heracles and was in fact his lifelong enemy! As for his name Alcides, Pindar tells us that he was so named in commemoration of the name of his reputed father's father Alcaeüs, with Heracles' father being Zeus.
As a name, too, Alcides echoes that of Heracles' mother, Alcmene, with its root alee meaning (most aptly for the great god) 'force', 'bodily strength'. (Compare other names starting Ale-.) Of course, there could well be a close link between the two names Alcides and Heracles, since the 'might' root (ale-) is present in both.
In his chapter on Christian names in Remains Concerning Britain, William Camden interprets the 'might' somewhat differently, and his entry for 'Hercules' runs: 'Gr. Glory, or illumination of the air, as it pleaseth Macrobius [a fourth-century Latin grammarian], who affirmed it to be proper to the Sun, but hath been given to valiant men for their glory.' (Macrobius was here working on aer, as mentioned for Hera, and cleos, 'fame', 'glory': in other words, he was dividing the name not as Her/acles but as Hera/cles.)
Before leaving the name, it seems worth pointing out that its meaning is almost certainly highly favourable, and that this is a characteristic of almost all names ending in -cles, even those of real people such as Sophocles (glory of wisdom) and Themistocles (glory of lawfulness). (Heracles' 'real' father, incidentally, was Amphitryon, the popular account of his birth stating that Alcmene had actually slept with Zeus when the latter was disguised as Amphitryon.)
The Roman version of the name has a derivation that is exactly the same as those already considered for Heracles: he seems to have been one of the earliest gods to have been imported from Greece - in fact, he may even have been the first.