WordNet records no synonym and no antonym for pencil. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a thin cylindrical pointed writing implement; a rod of marking substance encased in wood
graphite (or a similar substance) used in such a way as to be a medium of communication
“the words were scribbled in pencil”
Synonyms, antonyms and the broader and narrower relations for pencil come from Princeton University’s WordNet. Full sources and licences.
a figure formed by a set of straight lines or light rays meeting at a point
a cosmetic in a long thin stick; designed to be applied to a particular part of the face
“an eyebrow pencil”
write, draw, or trace with a pencil
“he penciled a figure”