If you add a raised fourth pitch to a seventh chord, the extension is referred to as a sharp 11.
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The #11 comes from counting scale degrees above the octave, implying the pitch is to be added to a seventh chord.
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The #11 is synonymous with the lydian mode (fourth mode of the major scale), which is a major scale with a raised fourth.
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A CMaj7#11 chord is spelled: C (1) E (3) G (5) B (7) F# (#11) and the corresponding scale would be C Lydian (the fourth mode of G Major).