E.C. number

Stands for 'Enzyme Commmission' Number¹. It generally looks something like:

3.4.23.4

Something like an IP address, only it has a single digit for the first two places ("n.n") and then one or more for the last two ("n.n.n.n" or "n.n.nn.n" or "n.n.n.nn" etc).

The point is to classify enzymes by their activity, so all "1.x.x.x" are oxidoreductases. The top level classes are:

1. Oxidoreductases
2. Transferases
3. Hydrolases
4. Lyases
5. Isomerases
6. Ligases

Although this won't make much sense to non-biochemists still reading, these broad groups divide the enzymes into reasonable sets which are then subdivided:

1. Second Level : Substrate/Donor Type
2. Third Level : CoSubstrate/Acceptor Type
3. Fourth Level : Specific Enzymes

So '1.1.1.1' is the oxidoreductase(1.*) that acts on Carbon-Hydroxyl groups(1.1.*), using NAD+/NADP+ (1.1.1.*) and is the specific enzyme Alcohol Dehydrogenase (1.1.1.1). Phew!

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¹ The 'Commmission' is a sinister group, established in 1956, trying to take over the world with enzymes!^2
2Not really.