1. Inclined to behave in an actively hostile fashion; such as, an aggressive regime.
2. Assertive, bold, and energetic: "The manager made an aggressive sales campaign."
3. Of or relating to an investment or approach to investing that seeks above-average returns by taking above-average risks.
4. Fast growing; tending to spread quickly and to invade; for example, an aggressive tumor.
5. Characterized by or inclined toward vigorous or intensive medical treatment: "The doctor believed it was necessary to take an aggressive approach to treating the infection."