Respected Sister Helen,
Allow me to have the honor of answering your question before Rumi :-)
There is a very subtle difference between 'Shaheed' & 'Shahid'. Both of them derive from SH-H-D = Carrying or procuring evidence = Witnessing = Testifying = Being visible = Being present = To be seen (vs Unseen - Ghayib).
'Shahid' and 'Shaheed' both mean a WITNESS. The word SHAHEED has not been used in the Qur'an for a martyr.
MAQTOOLEEN FI SABEELILLAH (Those who lay their lives in the Way of Allah) is the term used in the Qur'an for martyrs. [Singular will be MAQTOOL FI SABEELILLAH (a martyr), but I do not recall this form used in the Qur'an.]
So, SHAHID = Witness.
SHAHEED = Witness = SHAHID in an intense form = Someone who confirms his/her IMAN by giving one's life in the Way of Allah.
The word SHAHEED is very commonly used for a martyr in Hadith, FIQH (Islamic jurisprudence) and in common lingo, but not in the Qur'an.
The above post (very loosely) calls Benazir a 'Shaheed' or martyr. Who can say that she died in the Cause of Allah?