a widow: a woman whose husband has died
At its center this word means exactly what the English word "widow" means: a woman whose husband has died. That is its plain sense in almost every place it appears, from the poor widow who drops two small coins into the temple treasury to the widows weeping over Tabitha at Joppa.
But the word carries weight that the modern English word has lost. In the Old Testament and its Greek translation, the widow stands with the orphan and the foreigner as the picture of the person who has no one to protect her, the one God himself promises to defend. So when the New Testament speaks of widows it is usually not making a neutral note about marital status; it is naming the vulnerable, and calling God's people to the same care God shows. James makes this explicit: pure religion is to look after orphans and widows in their trouble.
In ordinary Greek the word could stretch a little wider than "bereaved by death." Because it comes from an old root meaning "left, forsaken, empty," it could sometimes cover a woman living without a husband for other reasons, such as separation or divorce. Whether the New Testament ever uses it that wide is debated, and the sharpest debate surrounds the instructions in 1 Timothy 5 about which widows the church should support and enroll.
The word can also be turned into a picture. A once-proud city stripped of its power and left desolate is called a widow, which is exactly the boast Babylon makes in Revelation when she says she will never sit as a widow.