An ohm is a unit of electrical resistance. It is the SI unit for electrical resistance. Mega is the SI prefix for 1 million. 1 megaohm is equal to 1 million ohms. The abbreviation for one megaohm is 1MΩ, making the preceeding equation 1MΩ = 1000000Ω.
A megaohm is made up of two components, the "mega" and the "ohm". The ohm is a unit that measures electrical resistance. Electrical resistance (of a wire, anyway) can be calculated through R = ρL/A, where R is the electrical resistance, ρ is the resistivity, L is the length of the wire, and A is the cross-sectional area of the wire. The ohm is an SI-derived unit, and it can be simplified into its composite base units, (kg·m^2)/(s^3·A^2), or kilograms times meters squared over seconds cubed times amperes squared. For general electric applications, Ohm's Law, V=IR where V is the voltage, I is the current, and R is the resistance, is followed.
The mega part is an SI prefix that indicated that the megaohm is 10^6 (or 1 million) times greater than an ohm. More precisely, it indicates that 1 megaohm is equal to 1,000,000 ohms. The abbreviation for ohm is omega (Ω), and the abbreviation for mega is M, so the previous equation can be rewritten as 1MΩ = 1000000Ω
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