question about electrical length of a center fed dipole

Im reading the ARRL 19th edition Antenna Book, chapter 2 of antenna fundamentals.

Under the section of "Impedance of a Center-Fed Dipole" it tells me that:

A center feed dipole can be any length electrically, as long as it is configured in a symmetrical fashion with two equal-length legs.

my question is what is the books meaning of "any electrical length". I suppose I am asking what defines "electrical length".

Is the electrical length just the physical length of the wire based on wavelength and frequency?

Please provide an example if you can.

There has to be more to the statement than:
A center feed dipole can be any length electrically, as long as it is configured in a symmetrical fashion with two equal-length legs.

There's many people here better than I concerning antennas but as an old ham, I've built a few 1/2 wavelength dipoles (also quad, yagi and LPAs). Possibly something about an antenna tuner?

I attached paragraph 1 and paragraph 2. The second paragraph has the passage that is in question.

yes well there is more to it

you have to take into context with the surrounding text.

Its stating that a 1/2 wave dipole is symetrical, as compared to other versions eg. offcentre fed dipoles

yes you can have ANY electrical (any length ) dipole BUT of course its going to be the length that is resonant for the freq you intend to use it on

Electrical length refers to the velocity factor of the material used for the dipole
as a close rule of thumb the electrical length of a dipole ( other antenna) that is bare metal... aluminium rod/copper wire is pretty much the same as the free space wavelength of the freq in use ( ~ 98 - 99%).
But if the metal is covered ... say in the case of insulated wire then the electrical length is some % below the freespace length
again this is ~ 95% of the freespace length.

cheers
Dave

http://en.wikipedia.org/wiki/Electrical_length