A Yiddish term that literally means "Year Time". Used to refer to commemorating the death of a loved one. Sometimes spelt Jahrzeit.
On the anniversary of the death of a loved one (parent, child, sibling or spouse), a Jewish person lights a memorial candle that burns for the entire Jewish day (ie it's lit at nightfall of the preceeding night and burns until nightfall the next day). Special candles are made that burn for about 25 hours. (Many people also light a Yahrzeit candle on Yom Kippur - see Yizkor.
(Many people have the tradition to commemorate the first Yahrzeit on the anniversary of the funeral rather than the death, but subsequent ones will be on the anniversary of the death).
It is seen as an appropriate time to give to charity in memory of the deceased. Orthodox men will go to Synagogue and recite the Kaddish prayer on the day. They may also be called up to the Kriat HaTorah (public reading of the Torah in Synagogue) on the Shabbat before the Yahrzeit, and the Rabbi may recite a prayer for the souls of the departed relatives members of the congregation who have a Yahrzeit in the following week.