Striving in order for the Word of Allah to remain supreme
Abu Musa Al-Ash’ari (radi Allahu anhu) reported that Messenger of Allah (sal Allahu alaihi wa sallam) was asked about he who fights in the battlefield out of valour, or out of zeal, or out of hypocrisy, which of this is considered as fighting in the cause of Allah? He (sal Allahu alaihi wa sallam) said: “He who fights in order that the Word of Allah remains supreme, is considered as fighting in the cause of Allah”
[Al-Bukhari and Muslim]
Since one’s action will be evaluated in terms of intention, so he alone (the who fights in order that the Word of Allah remains supreme) would be a Mujahid (warrior in the cause of Deen) who fights to glorify the Name of Allah.