In Altered Carbon, almost all characters refer to the ultra wealthy, powerful & famous as "Meths".
What exactly is a "Meth"?
Meths are essentially the richest of the rich, who have lived for a very long time because they can afford to continuously replace their sleeves.
The term comes from the word Methuselah,
In the Old Testament, Methuselah was the grandfather of Noah and the longest living person of all time.
In the show, billionaire Laurens Bancroft (James Purefoy) is one of the oldest living people ever.
From the novel it's explained as follows:
"Bancroft’s not people, like you and me. He’s a fucking Meth.”
“Yeah. A Meth. You know, and all the days of Methuselah were nine hundred sixty and nine years. He’s old. I mean, really old.”
Altered Carbon - Richard K. Morgan