About this character
The pound sign is a capital letter L with one or two horizontal strokes, derived from the Latin word libra, meaning "pound" in the sense of a unit of weight. Roman shopkeepers wrote lb. for one pound of mass (the abbreviation that still survives in English-speaking countries), and the same abbreviation came to designate one pound of silver as a unit of account. Mediaeval scribes drew it as an ornamented L, and by the 17th century English engravers and printers were treating it as a single typographic mark, evolving the modern form: a sloping L crossed by one stroke (sometimes two in older fonts) and with a curlicue terminal.
Today U+00A3 is used as the currency sign of the pound sterling — the official currency of the United Kingdom — and of several other "pound" currencies in the territories of the former British Empire and beyond: the Egyptian pound, the Lebanese pound, the Sudanese pound, the South Sudanese pound, the Syrian pound, and Saint Helena's pound. Historically it was the symbol for the Italian lira until the introduction of the euro in 2002 (the lira's name itself derives, via Italian, from libra; many fonts include the alternative glyph as U+20A4 ₤ LIRA SIGN).
U+00A3 is part of the original ISO 8859-1 set, so it has been encodable in plain text since the 1980s and has the short HTML named entity £. It must not be confused with the entirely separate U+0023 # NUMBER SIGN — which is also called "pound sign" in American English, where it refers to the unit of weight rather than the currency. This naming collision causes documentation confusion: in the US, "press the pound key" on a phone means #; in the UK, the same phrase would mean the currency symbol on a keyboard. The Unicode standard uses the unambiguous names POUND SIGN for £ and NUMBER SIGN for #.