Ombré is a gradient: one colour dissolving into another along the length of the nail with no visible seam. The beloved classic is the “baby boomer” — sheer pink at the cuticle fading to soft white at the tip, essentially a french manicure with the line airbrushed away — but the format takes any pair of colours that sit near each other in lightness.
Techs build ombré with sponges, ombré brushes or an airbrush; the craft is in the middle third, where the two colours must mix convincingly. It is a look that loves length (a gradient needs runway — across very short nails it reads as a smudge) and forgives grow-out well, since there is no hard line for regrowth to push upward. Related, softer cousins: aura's centred glow and glazed's pearl sheen.