> A tensor is a multidimensional array of numbers that transforms in the following way under a change of coordinates...

I saw that for years, and I <i>never</i> understood it until I saw the real definition of a tensor.

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<b>[Clarification]</b>
Sorry, I did leave that very vague.  A tensor is a multilinear function mapping some product of vector spaces $V_1\times \cdots \times V_n$ to another vector space.  In the context of differential geometry, we're really talking about a tensor *field*, which assigns a tensor to every point that acts on the tangent and/or cotangent spaces at the point.

A more abstract definition is possible by considering tensor products of vector spaces, but the definition using multilinear functions is (to me) extremely intuitive and general enough for a first encounter.  It also leads naturally enough to the abstract concepts anyway, as soon as you start thinking about the set of all tensors of a particular rank and its structure.

The "multidimensional array" definition suffers from conflating object and representation.  The array is an encoding of the underlying multilinear function, and it's perfectly reasonable if understood in that way (to partially reply to Scott Aaronson's comment).  Unfortunately, the encoding depends on an arbitrary choice (coordinate system), while the underlying function obviously doesn't, so it gets very confusing if you try to use it as the definition.

Regarding accessibility (also referring to Scott Aaronson's comment): I don't really agree: I think multilinear functions are pretty accessible.  Assuming a familiarity with vector spaces and linear transformations, multilinear functions are a natural and very tangible extension of those ideas.  And since multilinearity is *the* key concept underlying tensors, if you're going to deal with tensors, you should really just bite the bullet and deal with the concept.