1, The full form of m is Metre. It’s used on Academic & Science ,Units in Worldwide

Metre or meter (SI unit symbol: m), is the fundamental unit of length in the International System of Units (SI).

1 meter (m) = 100 centimeter (cm)

2, The full form of m is Minute. It’s used on Academic & Science ,Units in Worldwide

Minute (m or min.) is a period of time equal to 60 seconds or one-sixtieth (1/60) of an hour.

3, The full form of m is Mass (as in E=mc^2). It’s used on Academic & Science ,Physics in Worldwide

E=mc² is Albert Einstein’s Special Theory of Relativity. Where

E= energy, in Joules.

m= mass of the specified object, mass in Kilograms (1000 grams).

c= the speed of light.

4, The full form of m is Milli. It’s used on Academic & Science ,Units in Worldwide

Milli (symbol: m) is a unit prefix in the metric system denoting a factor of one-thousandth (1/1000). The prefix comes from the Latin mille, meaning one thousand (the plural is milia).

5, The full form of m is mobile. It’s used on Computing ,Websites in Worldwide

The letter m is sometimes used as a sub-domain for website to show that it is optimized for mobile devices.

Example: m.websitename.com

Here the m in the domain name means the website is optimized for mobile devices.