Middle names of two pres'dents,
and one of a time traveler,
A most famous sorceress,
and a programmer's laughter.
A call for assistance,
an invisible juncture.
What do they all mean?
(One word is the answer.)
Hint
Not looking for a pattern like one proposed by @cwallenpoole. All of the words described mean "something". What is that "something" ?
Hint 2
Edited first line of riddle