Shiur (//, Hebrew: שיעור [ʃiˈʔuʁ], pl. shiurim, שיעורים [ʃiʔuˈʁim] (Lit. Lesson, measure, term) is a lesson on any Torah topic, such as Gemara, Mishnah, halakha, Tanakh, etc.
The shiur has been a primary method of teaching since Mishnaic times. In a famous Talmudic passage, Rabbi Judah haNasi averred that he gained his sharp mind from watching Rabbi Meir deliver the shiur. However, since the lecture hall was so crowded, his seat was behind the lectern, so he only saw Rabbi Meir from the back. "Had I seen him from the front, how much greater would I have become!" he declared.
Traditionally, a shiur refers to the type of learning that takes place in yeshivot and kollelim, in which students hear an in-depth lecture on the sugya (Gemara topic) the yeshiva is studying at the time.
Typically, yeshiva students attend a daily shiur yomi (daily lecture) given by a maggid shiur (literally, "sayer of the shiur") and a weekly shiur klali (comprehensive lecture, which sums up the week's learning) given by the rosh yeshiva. The rosh yeshiva usually also gives the senior shiur - see below - on a daily basis.
Before the shiur, a bibliography and a series of textual references, or marei mekomot,   are posted so that students may prepare for the lesson in advance. Students typically spend several hours preparing for the shiur yomi. After the shiur, students spend additional time reviewing and clarifying the lesson that they have just heard. These preparation and review periods take place in a special time period called a seder, in which students study the lesson individually and/or in chavrutot (study pairs).
Shiurim may also be offered in yeshiva on topics in mussar, Chumash, and hashkafah (Jewish philosophy), depending on the yeshiva and the learning level of its students. The shiur is likewise the typical format for classes at women's seminaries and midrashot.
Especially in Chasidic settings, a less formal shiur may be termed "sicha" (שיחה, lit. "speech"); see also Maamor and Maamarim (Chabad).
A shiur is also the name given to the different class-levels in a yeshiva. For example, first-year students are said to be in "Shiur Aleph"; second-year students are in "Shiur Bet"; third-year students are in "Shiur Gimmel, etc. In kollelim, the higher shiurim accommodate more advanced levels of learning. See Yeshiva #Talmud study.
Synagogue rabbis and noted rabbis in the community also give shiurim to their constituencies. In shuls, the shiur given between Mincha and Maariv is usually geared to baalebatim (working men). Noted rabbis give more in-depth shiurim to attendees on Shabbat or weekday evenings, usually in the local synagogue or beth midrash (study hall).
Many yeshivot, midrashot, and "community kollels" host yemei iyun ("days of in-depth learning", sing. yom iyun), where community members study a specific topic. These are often held before religious holidays - especially Pesach and the New Year period - preparing the spiritual and halakhik elements of the upcoming festival.
In modern parlance, the term "shiur" has been extended to include any kind of Torah lesson — including lectures to children, women, and baalebatim (non-scholarly audience), and taped lectures circulated via cassette tape, computer hookup, MP3 or MP4 connection, or call-in telephone lines. Though by definition, a shiur is a comprehensive, structured lesson, some kiruv organizations advertise "five-minute shiurim" to attract beginning listeners.