You use definitely to emphasize that something is the case, or to emphasize the strength of your intention or opinion.
I'm definitely going to get in touch with these people.
我一定要联系上这些人。
While intra-region trade in Asia has definitely improved, the pace of recovery in individual economies has been uneven.
尽管亚洲区内贸易已有明显改善,但是单个经济体的复苏步伐仍不平衡。
If something has been definitely decided, the decision will not be changed.
She had definitely decided that she wanted to continue working with women in prison.
她打定主意要继续与监狱里的女人们一起工作。