The dictionary defintion of optimal is just 'best'; the connotation that 'optimal' means not perfect, but satisfying multiple constraint, is engineering jargon. In fact, with 'optimal', people will often mean 'Pareto optimal' (a design where there is no change that would improve the solution on all dimensions/constraints).

Pareto optimality also implies that multiple designs may exist in a 'draw', where design 1 is better on dimension A, but worse on dimension B, and design 2 flips those. These designs are on a pareto front. Making the trade-offs explicit helps readers of the design document who come along later to choose a different trade-off when it later turns out that the relative importance of the dimensions involved changed.