A strongly saturated, intense colour — the opposite of muted.
Colourwise classifies a colour as vivid when HSV saturation is 70% or higher.
Vivid is about saturation, not lightness — a vivid colour can be light (bright yellow) or dark (deep red).
Traffic-cone orange and emerald green are vivid colours.
Vivid accents are the usual choice when a 'bold' rather than 'subtle' pairing is wanted.