It's called doubling. The term is correctly used in the OP: in the intro to "Crosstown Traffic", Jimi doubles the vocal melody on guitar (or the guitar melody on voice).
- When an accompanying instrument plays the same notes that a singer is singing. (Source: OnMusic Dictionary)
Wikipedia also discusses the term in its article on voicing in music:
Octave doubling of a voice or pitch is a number of other voices duplicating the same part at the same pitch or at different octaves. (Source: Wikipedia)
The term is used across styles of music.
One example from classical music — both of the technique itself and the use of the term — comes from Liszt's song "Comment, disaient-ils" for voice and piano (S.276). In measures 12 – 18, the vocal melody is doubled by the top voice of the piano part.
Here is a description from a dissertation discussing some of Liszt's songs:1
In the original version, the descending melody in the voice is doubled in the piano over a waltz-like left-hand accompaniment. (Source: "Franz Liszt's Songs on Poems by Victor Hugo", by Shin-Young Park, PhD diss., Florida State University, 2007, p. 20)
(Image source: IMSLP)
Doubling can also refer to 1) a note that appears more than once within a chord, or 2) a musician who plays more than one instrument (James Moody played tenor sax and doubled on flute).
1 A couple of other examples of the term's use: 1) "The voice is doubled in the piano for the first time in the song in mm. 21-22, expressing the unity of our couple." ("Glances: an Analysis of the Song Cycle by Tom Cipullo", Renee Mae Clair, PhD diss., University of Memphis, 2011, p. 50); 2) "Then, for the last few lines of text, though the chords in the piano remain dissonant, the voice is doubled in the piano chords." ("Song Cycles for Soprano by Richard Pearson Thomas", Laura Faith Bateman, PhD diss., University of Northern Colorado, 2011, p. 74)