Someone designs thing X at time t if and only if:

1. She imagine or describes X at t
2. While supposing that X at least partially satisfies some set of requirements R under conditions C and
3. Satisfying R is a problem for which
4. X is a novel or original solution

(TPOD p10 ¶2 L5)

paraphrased:

design is the intentional solution of a problem by the creation of plans for a new sort of thing […] we can now define the noun “design” as the problem-solving plan generated by this activity.

(TPOD p10 ¶4 L3)