The word mourn in Matthew 5:4, "Blessed are they that mourn for they shall be comforted" is defined thus by Strong's dictionary of the New Testament:
πενθέω pentheō to grieve (the feeling or the act): - mourn, (be-) wail.
Here is the definition from Vine's New Testament Dictionary:
pentheo (G3996), "to mourn for, lament," is used (a) of mourning in general, Mat_5:4; Mat_9:15; Luk_6:25; (b) of sorrow for the death of a loved one, Mar_16:10; (c) of "mourning" for the overthrow of Babylon and the Babylonish system, Rev_18:11, Rev_18:15, RV, "mourning" (KJV, "wailing"); Rev_18:19 (ditto); (d) of sorrow for sin or for condoning it, Jam_4:9; 1Co_5:2; (e) of grief for those in a local church who show no repentance for evil committed, 2Co_12:21, RV, "mourn" (KJV, "bewail").
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