In 54 cases, the word is used to mean evidence, witness, present and aware, and in one case it means a person killed on the path of God.
The word Shaheed has different meanings. One of them is a person to whose knowledge nothing is hidden. Obviously, it is only God who can have such an attribute Bil-Dhat (by essence). That is why it is mentioned in many verses of the Quran as an attribute of God:
“Say: ‘People of the Book, why do you disbelieve the verses of Allah? Surely, Allah is witness to all that you do.’” (Verse 98 of Surah Al Imran)
It the next stages are God’s messengers and the angels:
“And so We have made you a median nation, in order that you will be a witness above the people, and that the Messenger be a witness above you.” (Verse 143 of Surah Al-Baqarah)
“Every soul will be accompanied (by an angel) behind him and another as a witness.” (Verse 21 of Surah Qaaf)
So Shaheed sometimes means witness. For example in Surah Al-Baqarah, Verse 282, witnesses to a deal are called Shaheed: “…let no harm be done to either scribe or witness.”
Sometimes it means being present: “… therefore whoever of you is present in the month…” (Verse 185 of Surah Al-Baqarah)
And sometimes it means a guide and an example. (Verse 143 of Surah Al-Baqarah)
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The only verse in which the word Shaheed means a martyr killed on the path of God is Verse 69 of Surah An-Nisa: “Whosoever obeys Allah, and the Messenger, they are with those whom Allah has favored, the Prophets, the sincere, the martyrs and the righteous, and these are the best company.”
Of course, since the lifetime of the Holy Prophet (PBUH), the word has been used in this sense a lot.
Maybe a person killed on the path of God is called a Shaheed because he has sacrificed his life to help the religion of God, thus bearing witness to the fact that God and His commands are the truth and all others are falsehood. Such a person who bears witness to the truth by sacrificing his life in this world will have the merit to be among those who bear witness to others: “Allah bears witness that there is no god except He, and so do the angels and the knowledgeable.” (Verse 18 of Surah Al Imran)