What the omega symbol means
Omega (Ω, small ω) is the 24th and last letter of the Greek alphabet. Its name means "big O" (o mega), as opposed to omicron, "small o". Because it comes last, omega stands for the end or the final stage, and "the Alpha and the Omega" means the beginning and the end. You will often see Α and Ω together in churches and Christian art.
In science, Ω is the unit symbol for the ohm, the unit of electrical resistance; see the ohm symbol page for kΩ, MΩ and circuit symbols. Small ω is angular velocity or angular frequency, ω = 2πf. In probability, Ω is the sample space, in cosmology it is the density parameter, and in computer science Ω(n) gives a lower bound on running time.
In nutrition, omega-3 and omega-6 fatty acids are named for where their first double bond sits, counted from the omega (last) end of the carbon chain.
Omega vs. the ohm sign Ω
Unicode has a separate Ω ohm sign (U+2126), kept for compatibility with older character sets. It is defined as equivalent to the Greek capital omega, Unicode normalization turns it into Ω, and Unicode recommends the Greek letter Ω (U+03A9) even when you mean ohms.
Other look-alikes: ℧ is the mho, an upside-down ohm sign once used for conductance (now measured in siemens). ɷ is a closed omega from older phonetic notation, and ϖ is a variant form of pi that looks like an omega with a bar on top.
How to type Ω and ω
On a Mac with the US layout, Option + Z types Ω. On Windows, hold Alt and type 234 on the number pad. Small ω has no Alt code: in Word, type 03C9 and press Alt + X, or insert an equation and type \omega.
With the Greek keyboard layout, on a computer or a phone, the V key types ω. In LaTeX, use \Omega and \omega; in HTML, Ω and ω. The first letter of the alphabet, omega's partner in "Alpha and Omega", is on the alpha symbol page.