Difference between DECIMAL and NUMERIC

What's the difference between the SQL datatype NUMERIC and DECIMAL ? If databases treat these differently, I'd like to know how for at least:

SQL Server
Oracle
Db/2
MySQL
PostgreSQL

Furthermore, are there any differences in how database drivers interpret these types?

They are the same for almost all purposes.

At one time different vendors used different names (NUMERIC/DECIMAL) for almost the same thing. SQL-92 made them the same with one minor difference which can be vendor specific:

NUMERIC must be exactly as precise as it is defined — so if you define 4 decimal places to the left of the decimal point and 4 decimal places to the right of it, the DB must always store 4 + 4 decimal places, no more, no less.

DECIMAL is free to allow higher numbers if that's easier to implement. This means that the database can actually store more digits than specified (due to the behind-the-scenes storage having space for extra digits). This means the database might allow storing 12345.0000 in the above example of 4 + 4 decimal places, but storing 1.00005 is still not allowed if doing so could affect any future calculations.

Most current database systems treat DECIMAL and NUMERIC either as perfect synonyms, or as two distinct types with exactly the same behavior. If the types are considered distinct at all, you might not be able to define a foreign key constrain on a DECIMAL column referencing a NUMERIC column or vice versa.

They are synonyms, no difference at all.

At least ~~on SQL Server~~ in the ANSI SQL standards. This SO answer shows some difference in ANSI but I suspect in implementation they are the same

Postgres: No difference

in documentation description in table 8.1 looks same, yet it is not explained why it is mentioned separately, so according to Tom Lane post

There isn't any difference, in Postgres. There are two type names because the SQL standard requires us to accept both names. In a quick look in the standard it appears that the only difference is this:
     17)NUMERIC specifies the data type exact numeric, with the decimal
        precision and scale specified by the <precision> and <scale>.

     18)DECIMAL specifies the data type exact numeric, with the decimal
        scale specified by the <scale> and the implementation-defined
        decimal precision equal to or greater than the value of the
        specified <precision>.
ie, for DECIMAL the implementation is allowed to allow more digits than requested to the left of the decimal point. Postgres doesn't exercise that freedom so there's no difference between these types for us.
      regards, tom lane

also a page lower docs state clearly, that

The types decimal and numeric are equivalent. Both types are part of the SQL standard.

and also at aliases table decimal [ (p, s) ] is mentioned as alias for numeric [ (p, s) ]

They are actually equivalent, but they are independent types, and not technically synonyms, like ROWVERSION and TIMESTAMP - though they may have been referred to as synonyms in the documentation at one time. That is a slightly different meaning of synonym (e.g. they are indistinguishable except in name, not one is an alias for the other). Ironic, right?

What I interpret from the wording in MSDN is actually: These types are identical, they just have different names.

Other than the type_id values, everything here is identical:

SELECT * FROM sys.types WHERE name IN (N'numeric', N'decimal');

I have absolutely no knowledge of any behavioral differences between the two, and going back to SQL Server 6.5, have always treated them as 100% interchangeable.

for DECIMAL(18,2) and NUMERIC(18,2)? Assigning one to the other is technically a "conversion"?

Only if you do so explicitly. You can prove this easily by creating a table and then inspecting the query plan for queries that perform explicit or - you might expect - implicit conversions. Here's a simple table:

    CREATE TABLE [dbo].[NumDec]
(
    [num] [numeric](18, 0) NULL,
    [dec] [decimal](18, 0) NULL
);

Now run these queries and capture the plan:

DECLARE @num NUMERIC(18,0);
DECLARE @dec DECIMAL(18,0);

    SELECT 
      CONVERT(DECIMAL(18,0), [num]), -- conversion
      CONVERT(NUMERIC(18,0), [dec])  -- conversion
    FROM dbo.NumDec
    UNION ALL SELECT [num],[dec] 
      FROM dbo.NumDec WHERE [num] = @dec  -- no conversion
    UNION ALL SELECT [num],[dec] 
      FROM dbo.NumDec WHERE [dec] = @num; -- no conversion

we have explicit conversions where we asked for them, but no explicit conversions where we might have expected them. Seems the optimizer is treating them as interchangeable, too.

Personally, I prefer to use the term DECIMAL just because it's much more accurate and descriptive. BIT is "numeric" too.

Difference between DECIMAL and NUMERIC

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