'Shaheed' is someone who has taken 'shahada'. The direct translation of shahada is 'witness'. Traditionally, when someone takes the oath to become a muslim he/she bears witness that there is no god but Allah and Muhammed pbuh is Allah's messenger. Thus, technically, all muslims should be shaheed.
So why are only martyrs refereed to as shaheed while a muslim who dies a regular non-jihad death generally is not referred to or remembered as a shaheed. What are the martyrs witnessing as they die from battle?