Definition: the weighed balance of ground almonds, icing sugar, egg whites and caster sugar that a French macaron depends on.
Macarons are aratio recipe, not a “two eggs” one — which is exactly why weighing matters so much. A classic starting point is equal weights of ground almonds and icing sugar, with egg whites and caster sugar close behind (in my recipe, 100g / 100g / 75g / 75g). Because eggs vary, “one white” isn’t precise enough to trust; a gram or two out shifts the whole batter. Weigh everything, including the whites.
In practice: use digital scales and weigh the whites into a bowl — don’t count them.
Related:macaronage,French meringue.