An adverb meaning 'by all means' or 'certainly'; used to express strong recommendation or desire.
来週の歓迎会、ぜひ参加してください。
Please be sure to attend next week's welcome party.
この本は面白いから、ぜひ読んでみて。
This book is interesting — definitely give it a read.
ぜひ一度お目にかかりたいです。
I would very much like to meet you once.
A noun meaning 'right and wrong' or 'pros and cons' — used to discuss the correctness or desirability of something.
この問題の是非を明らかにする必要がある。
It is necessary to clarify the pros and cons of this issue.
裁判は事件の是非を判断する立場にある。
The judge is in a position to determine the right and wrong of the case.
君の提案の是非についてチームで話し合おう。
Let's discuss the pros and cons of your proposal with the team.