'FTSE 250' is explained in detail and with examples in the Investments edition of the Herold Financial Dictionary, which you can get from Amazon in Ebook or Paperback edition.
FTSE 250 is a broad-based stock index maintained by the FTSE company. This company is much like Standard and Poor’s in that they both concentrate their efforts on calculating indices. The FTSE is not made up of any stock exchange, though among its co-owners is the famed and historic London Stock Exchange (LSE). The other co-owner of the company is its namesake the Financial Times newspaper publishing empire.
Easily the best known of the FTSE indices is the FTSE 100. There are many thousands of indices owned, produced, and calculated by FTSE, but only one is the blue chip index of all British and international company and economy stocks based on the LSE. The second most important and widely cited index from the company is this FTSE 250. This one is made up of the 101st to 351st largest companies in the U.K. As these firms tend to be much more British and far less international than those making up the FTSE 100, they are gauged to be a superior measurement of how the British economy and U.K. based firms are actually performing.
The FTSE 250 index list is altered four times a year on a quarterly basis. This occurs reliably every March, June, September, and December month. The index itself is continuously calculated instantly in real time. The owner of the index publishes it every minute accordingly online and through financial news and media outlet feeds.
As of November 7, 2016, the 350 different constituent companies which comprise the FTSE 250 are as follows: 3i Infrastructure, AA, Aberdeen Asset Management, Aberforth Smaller Companies Trust, Acacia Mining, Aggreko, Aldermore Group, Alliance Trust, Allied Minds, Amec Foster Wheeler, AO World, Ascential, Ashmore Group, Assura, WS Atkins, Auto Trader Group, Aveva, Balfour Beatty, Bankers Investment Trust, Barr, A.G., BBA Aviation, Beazley Group, Bellway, Berendsen, Berkeley Group Holdings, Bank of Georgia Holdings, BH Macro, Big Yellow Group, B & M European Retail Value, Bodycote, Booker Group, Bovis Homes Group, Brewin Dolphin Holdings, British Empire Trust, Britvic, Brown N, BTG, Cairn Energy, Caledonia Investments, Capital & Counties Properties, Card Factory, Carillion, Centamin, Cineworld, City of London Investment Trust, Clarkson, Close Brothers Group, CLS Holdings, CMC Markets, Cobham, Computacenter, Countryside Properties, Countrywide, Cranswick, Crest Nicholson, CYBG, Daejan Holdings, Dairy Crest, Debenhams, Dechra Pharmaceuticals, Derwent London, DFS, Dignity, Diploma, Domino’s Pizza, Drax Group, Dunelm Group, Edinburgh Investment Trust, Electra Private Equity, Electrocomponents, Elementis, Entertainment One, Essentra, Esure, Euromoney Institutional Investor, Evraz, F&C Commercial Property Trust, Fidelity China Special Situations, Fidelity European Values, Fidessa Group, Finsbury Growth & Income Trust, FirstGroup, Fisher, James & Sons, Foreign & Colonial Investment Trust, G4S, Galliford Try, GCP Infrastructure Investments, Genesis Emerging Markets Fund, Genus, Go-Ahead Group, Grafton Group, Grainger, Great Portland Estates, Greencoat UK Wind, Greencore, Greene King, Greggs, GVC Holdings, Halfords Group, Halma, Hansteen Holdings, HarbourVest Global Private Equity, Hastings Group, Hays, Henderson Group, HICL Infrastructure Company, Hill & Smith, Hiscox, Hochschild Mining, Homeserve, Howden Joinery, Hunting, Ibstock, ICAP, IG Group Holdings, IMI, Inchcape, Indivior, Inmarsat, Intermediate Capital Group, International Personal Finance, International Public Partnerships, Investec, IP Group, Jardine Lloyd Thompson, JD Sports, John Laing Group, John Laing Infrastructure Fund, JPMorgan American Investment Trust, JPMorgan Emerging Markets Investment Trust, JPMorgan Indian Investment Trust, JRP Group, Jupiter Fund Management, Just Eat, KAZ Minerals, Keller, Kennedy Wilson Europe Real Estate, Kier Group, Ladbrokes Coral, Laird, Lancashire Holdings, LondonMetric Property, Man Group, Marshalls, Marston’s, McCarthy & Stone, Meggitt, Mercantile Investment Trust, Metro Bank, Millennium & Copthorne Hotels, Mitchells & Butlers, Mitie, Moneysupermarket.com Group, Monks Investment Trust, Morgan Advanced Materials, Murray International Trust, National Express Group, NB Global, NCC Group, NMC Health, Ocado Group, OneSavings Bank, P2P Global Investments, PageGroup, Paragon Group of Companies, PayPoint, Paysafe, Pennon Group, Perpetual Income & Growth Investment Trust, Personal Assets Trust, Petra Diamonds, Petrofac, Pets at Home, Phoenix Group Holdings, Playtech, Polar Capital Technology Trust, Polypipe, PZ Cussons, QinetiQ, Rank Group, Rathbone Brothers, Redefine International, Redrow, Regus, Renewables Infrastructure Group, Renishaw, Rentokil Initial, Restaurant Group, Rightmove, RIT Capital Partners, Riverstone Energy, Rotork, RPC Group, Safestore, Saga, Savills, Scottish Investment Trust, Scottish Mortgage Investment Trust, Segro, Senior, Serco, Shaftesbury, Shawbrook Bank, SIG plc, Smith (DS), Smurfit Kappa Group, Softcat, Sophos, Spectris, Spirax-Sarco Engineering, Spire Healthcare, Sports Direct, SSP Group, Stagecoach Group, St. Modwen Properties, SuperGroup, SVG Capital, Synthomer, TalkTalk Group, Tate & Lyle, Ted Baker, Telecom Plus, Temple Bar Investment Trust, Templeton Emerging Markets Investment Trust, Thomas Cook Group, Tritax Big Box REIT, TR Property Investment Trust (two listings, both ordinary & sigma shares), Tullett Prebon, Tullow Oil, UBM, UDG Healthcare, UK Commercial Property Trust, Ultra Electronics Holdings, Unite Group, Vectura Group, Vedanta Resources, Vesuvius, Victrex, Virgin Money, Weir Group, Wetherspoon (J D), W H Smith, William Hill, Witan Investment Trust, Wizz Air, Woodford Patient Capital Trust, Wood Group, Workspace Group, Worldwide Healthcare Trust, and Zoopla.