[amirmc]

There's no fixed definition and I've found that people use it when they're asking for 'broad' skills -- where broad is defined by their own experience (cf. most of the other comments here). For example, most web developers would consider front-end and back-end as 'full-stack', whereas an OS/systems person might consider knowledge of hardware up through kernels into the application layer as 'full-stack'.

It's about as useful as the term 'Ninja' or 'Rockstar' in that it's used when people can't articulate succinctly what they mean (or they don't really want to take the time to do so).

[guan]

It should really include hardware and electronics down to the transistor level, and higher level skills like UI, architecture, marketing.

[ecaradec]

it should really go down to quantum physic, thermodynamic and up to philosophy...

[ethbro]

It all collapses down to a figment of my imagination anyway.

(Btw, now hiring full solipsist developers. I'll find you if I'm interested)