in an important way or to an important degree
"more importantly, Weber held that the manifold meaning attached to the event by the social scientist could alter his definition of the concrete event itself"
In a significant manner or to a significant extent.
"The new marketing campaign significantly increased our website traffic by over fifty percent last month."
In plain English: Significantly means something is big enough to really matter or make a noticeable difference.
"The new policy has significantly reduced traffic in the city center."
Usage: Use significantly when describing an action that causes a large, noticeable change in quantity or degree rather than just being important. It functions as an intensifier similar to greatly but specifically emphasizes measurable magnitude over general importance.
The word significantly comes from combining the adjective significant with the suffix -ly to form an adverb. It entered English by adding a common grammatical ending that indicates manner or degree to the existing root word.