The Hebrew noun tav means a mark or sign — literally, a scratch, cross-mark, or brand used to mark ownership or identity. In Ezekiel 9:4, God commands an angel to place a tav on the foreheads of those who grieve over Jerusalem's abominations, marking them for preservation. This is one of Scripture's most powerful 'sealing' passages.
The tav mark in Ezekiel 9 carries enormous theological weight. In ancient Paleo-Hebrew script, tav was written as an 'X' or cross-mark — leading early Christians to see a prefigurement of the cross of Christ as the mark that preserves God's people. Whether or not this was Ezekiel's intent, the theological pattern is profound: God places a distinguishing mark on His faithful remnant to preserve them through judgment. This anticipates Revelation's seal of the 144,000 (Rev 7:3) and the name of the Lamb on the foreheads of the redeemed (Rev 14:1). Belonging to God is visibly, permanently marked.