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Affogato: Espresso poured over icecream.

60ml shotglass
Vanilla ice-cream
Shot of espresso. (30ml).

(With thanks to E. Vinas.)

Boiler: Espresso machine boilers can range in size from 120ml in domestic machines to 12 litres and over in commercial machines. Lower end domestic espresso machines and domestic superautomatic machines often don't have boilers at all, instead using thermoblocks.
Burrs: Burrs used in espresso grinders can be flat, as in the picture, or conical. They MUST be sharp in order to provide the precision necessary for correct espresso brewing. Cheap burr grinders sold at appliance stores are unable to cope with quality espresso machines.
Cafe Crema: A long espresso drink (150 - 180ml) produced using at least 14g of coarser ground coffee in the double filter basket for a total shot time of 20 - 25 seconds.
Cafe Latte: An espresso based drink consisting of a single espresso shot topped up with hot microfoamed textured milk and finished with no more than 1cm of foam on top.
Cappuccino: An espresso based drink consisting of one-third espresso, one-third hot milk and one-third milk foam, sometimes topped with chocolate powder. Normally made with a double espresso, a weak cappuccino can be made with a single espresso and the volume topped up with milk.
Descalers: Espresso machine descalers are organic acids (such as citric, tartaric and sulfonic acids) which dissolve calcium and magnesium carbonates and sulfates  deposited inside espresso machine boilers and the brew water path. They are normally "food safe" and are added to the incoming water supply. The machine is then flushed to remove both the acids and the scale. Descalers are not suitable for backflushing or cleaning.
Detergents: Espresso machine detergents are generally based on tri-sodium phosphate, and are used for backflushing (with the blind filter) and cleaning exterior machine parts like drip trays and portafilters. They must NEVER be added to the tank or boiler, as they will cause serious contamination and may damage sensors and valves.
E-61 Group. Originally patented by Faema, this group is a massive (4kg) chunk of chromed brass, designed to act as a heat radiating temperature stabilizer as part of a heat exchanger water circulation system. It brought unprecedented temperature control (and style) to the commercial espresso machine. Currently starting to appear on "semi-commercial" domestic machines as well.
Espresso: 28 2.5ml of coffee produced from 7 0.5g of finely ground coffee extracted at a pressure of 9  1bar at a temperature of 90 3C for a time of 25 2.5 seconds. Should be rich, unctuous, bittersweet in flavour.
Grinder: Commercial espresso grinders are "Burr Grinders" and almost always come with a doser attached to them, see grinder at left. Domestic grinders can also have dosers, or be doserless, as with the grinder at right. All espresso grinders come with rests ("doser forks") for the portafilter.
Filter baskets: Stainless steel baskets of various size, shape and depth (depending on machine) with holes in the base. Ground coffee is dosed into the basket, which is held in the portafilter, and the assembly  is then locked into the group before brewing commences. A basket with no holes (left) is called a blind filter, and is used for backflushing machines with 3 - way valves.
Group: (Gruppo in Italian.) Sometimes called the brew-head. The business end of the machine; the bit the portafilter locks into, and from which the pressurized hot water is forced through the coffee in the filter basket. Consists of a metal cylinder with passages in it for water, sometimes also has various valves built in. Surrounded by the group gasket and the locking collar which hold the portafilter in place, and topped with the shower screen which distributes the water.
Group Gasket: The rubber ring inserted in the group to ensure that the portafilter and filter basket combination seal tightly, stopping leaks. Group gaskets harden with age and should be replaced when they no longer seal properly.
Macchiato: An espresso based drink consisting of a single espresso shot in a small glass, with a dash of cold milk and a dot of hot foam added to the top.
Portafilter: Also called (incorrectly) "Group Handle". The carrier which holds the filter basket, and which locks into the group to provide a watertight seal for the pressurized water to infuse the coffee. The espresso then exits the portafilter via the spout(s). Portafilters normally have single or double spouts, but triple spout versions do exist. Domestic machine portafilters always have 2 spouts.
Puck: The puck is the disc shaped cake of coffee left behind after brewing. It should be firm, cohesive and dry when knocked out of the portafilter.
Rotary Pump: Most commercial espresso machines use rotary pumps connected to an electric motor. The most common brand is Procon. The pumps themselves are fairly small, but the electric motors are quite big.
Showerscreen: In most domestic machines the showerscreen is a simple perforated stainless steel disk which distributes the water from the group onto the coffee in the filter basket. In high end and commercial machines the disk is 2 - part; a solid stainless steel disc back with relatively large holes and a fine stainless steel mesh front, which provides a more even water distribution. 
Solenoid 3 - way Valves: These valves serve to relieve the built up pressure in the group and portafilter assembly immediately after brewing. They are normally actuated electronically via a solenoid, but mechanical versions of this valve (as on the original Faema E-61 group) also exist. They are present in high end domestic machines and all commercial machines. ONLY machines with these valves can be backflushed.
Tamper: The Tamper is a device for pressing the coffee into the filter basket, so that there are no voids in the coffee and the pressurized water cannot find the path of least resistance (channelling) at the edges of the coffee. There is some debate over how critical tamper size, shape and pressure is, but in my experience correctly ground coffee requires only a light tamp.
Thermoblock: A metal block with a large number of narrow channels inside, heated by an external element. Used in low end domestic espresso machines. Picture shows an opened thermoblock.
Vibration Pump: Pump domestic espresso machines and some of the lower end small commercial machines use vibration pumps, most of them produced by ULKA in Italy.

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