What the ohm symbol means
The ohm (Ω) is the SI unit of electrical resistance, named after the German physicist Georg Simon Ohm. Ohm's law ties it to voltage and current: R = V / I, so a resistor that lets 1 ampere flow at 1 volt has a resistance of 1 Ω.
Prefixes scale it: mΩ (milliohm) for tiny contact resistances, and kΩ (kilohm, a thousand ohms) and MΩ (megohm, a million ohms) for typical resistors. Parts lists and schematics also use letter codes such as 4k7 for 4.7 kΩ and 220R for 220 Ω, which avoid both the Ω and an easily lost decimal point.
The upside-down ℧ is the mho, an old unit of conductance (ohm spelled backward). Today conductance is measured in siemens (S).
Ohm sign vs Greek omega
Unicode has two Ω characters. Ω (U+03A9) is the Greek capital letter omega, and Ω (U+2126) is a separate ohm sign kept for compatibility with older character sets. They are canonically equivalent, so normalization turns the ohm sign into omega, and Unicode recommends the Greek letter U+03A9 for ohms. That is the one this page copies by default.
Lowercase omega ω is something else: in electronics it usually means angular frequency (ω = 2πf). For the letter itself and its other meanings, see the omega symbol page.
How to type Ω
Mac: Option + Z types Ω. Windows: hold Alt and type 234 on the number pad, or in Word type 03A9 and press Alt + X. HTML: Ω. LaTeX: \Omega in math mode, or \si{\ohm} with the siunitx package.
Phones have no Ω key, but you can add a Greek keyboard or copy Ω from this page. Where Ω might not display, such as some parts databases or old terminals, write "ohm" or use the R notation instead. For the other prefixes you will meet in datasheets, see the micro symbol page.