The model that matters is Christina Maslach's, with three distinct dimensions: emotional exhaustion (the tank is empty), depersonalisation or cynicism (distance from people, who become cases or tickets) and reduced personal accomplishment (the sense that nothing you do counts).
The practically important detail: the three do not arrive together. Exhaustion comes first, cynicism is a defence against it, and the loss of efficacy comes last and is the hardest to reverse. Intervening at the first sign is a far shorter road.
In ICD-11 the WHO classified burnout as an occupational phenomenon — not a medical condition. The distinction is not bureaucratic: it says the problem is located in the relationship with work, and an intervention treating only the person while leaving the context unchanged usually produces relapse.
Which is why serious recommendations include changes to workload, control over the work, recognition and fairness — not only relaxation techniques for the exhausted person.