The term "eye-bag" is frequently used to describe a series of under-eye conditions, and is often a misnomer for some of them.
The true "eye-bag" refers to an actual paeudo-herniation of the infra-orbital fat - the fat under the eye pushes against a fibrous wall that is meant to keep it from spilling out. This happens due to the gradual weakening of that orbital septum, together with changes in soft tissue and the bony structure of the facial skeleton over time (Fig 1.). Often though, the fat herniation could very well be congenital and be present from youth.
The rest of us typically suffer from one of the following issues that might not actually involve the true formation of "eye-bags":
Dark brown under-eye pigmentation: Due to accumulation of pigmentation that can be congenital or due to the accumulation of sun damage over the years
A bluish/purplish hue: Due to the visibility of vessels at the under-eye area, due to a combination of factors including genetic predisposition and thin skin
Despite the seemingly clear classification of the types of under-eye problems above, many of us suffer from a combination of these issues. Inevitably then, a multi-pronged approach would be necessary to tackle the problem satisfactorily.
True, untrue, or mixed eye-bag?
Optimal results typically require a combination of procedures, especially if several issues exist. These treatments can be spaced apart, and some can be done together. Some treatments (such as AGNES eye-bag reduction or surgery) are a one time affair, but treatments to reduce pigmentation and blend the lower eye (fillers, chemical peels, lasers) are typically required on a regular basis to maintain best results.