The word satan simply means adversary. It mostly refers to our lower self being our satan.
When the ungodly curseth Satan, he curseth
his own soul.
Jesus Sirach 21;27
There is also mention of satans, followers of Satan, demons guided by that lower self
There are lots of satans.
And a command has gone forth from the presence of the Lord concerning those who dwell on the earth that their ruin is accomplished because they have learnt all the secrets of the angels, and all the violence of the satans, and all their powers -the most secret ones- and all the power of those who practice sorcery, and the power of witchcraft, and the power of those who make molten images. 1Enoch 65;6
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and they follow what the satans recited over Solomon's kingdom. Solomon disbelieved not, but the satans disbelieved, teaching the people sorcery. Quran S. 2:106
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He (Mani) said: 'Then Jesus came and spoke to the one who had been born, who was Adam, and explained to him (about) the gardens (of Paradise), the deities, Gehenna, the satans, earth, heaven, sun, and moon. Ibn alNadim, Fihrist (a pud Flugel, Mani 58-61)
In a midrash (Genesis Rabbah 19) Samael, the lord of the satans, once was a mighty prince of angels in heaven who turned evil and became our greatest enemy.
Samael was a leader (read; leading group) of demons.