Decidedly means clearly, definitely, and without any doubt. When something is decidedly true, there's no question about it. If your teacher says your essay is decidedly better than your first draft, she means the improvement is obvious and unmistakable, a major leap forward.

The word adds emphasis and certainty. A room might be decidedly cold, meaning everyone can feel it's genuinely cold, far beyond sort of chilly. A flavor might be decidedly sweet, leaving no doubt about its sweetness. When a basketball team is decidedly superior to its opponents, it's clearly and noticeably the better team, winning through obvious skill.

Decidedly comes from the word “decide,” and it carries that sense of something being settled or determined. When you use it, you're telling your listener that there's no room for debate or uncertainty. The evidence is clear, the difference is obvious, and the conclusion is firm.