1) A characteristic attributed usually to non-native plants that humans really, really, really don’t like.
2) The ability of certain plants (usually non-natives) to exude chemicals into the soil that suppresses the growth of other plants (usually native ones) under laboratory conditions, but not in the real world.
3) A ten-dollar word used to scare people into taking action against non-native plants that have human-like evil intent, such as purposefully poisoning, shading, and/or smothering innocent spring ephemerals (only very, very bad invasive plants can achieve this trifecta).