1) Generically on flags, a (relatively) narrow band or line of contrasting colour separating two areas of similar or differing colour, such as a band, charge or canton, from its field (see also ‘ border'). 2) Specifically and in heraldry, as above but a narrow band or line of white/silver (argent) or yellow/gold (or) placed on a shield, banner of arms or flag in accordance with the rule of tincture (see also ‘ argent', ‘ border', ‘ or', ‘ multi-stripe' and ‘ rule of tincture').
In heraldry and vexillology, fimbriation refers to small strips of colour (technically called "tincture" in this sense in heraldry) placed around common charges or ordinaries, usually in order for them to stand out from the background, but perhaps just because the designer felt it looked better, or for a more technical reason (in heraldry only) to avoid what would otherwise be a violation of the rule of tincture.