DiracSlash[p] is the contraction p^{\mu } \gamma _{\mu } (FV[p, mu] GA[mu]).
Products of those can be entered in the form GS[p1, p2, …].
The shortcut DiracSlash is deprecated, please use GS instead!
This is q-slash, i.e., \gamma^{\mu} q_{\mu }
DiracSlash[q]\bar{\gamma }\cdot \overline{q}
DiracSlash[p] . DiracSlash[q]\left(\bar{\gamma }\cdot \overline{p}\right).\left(\bar{\gamma }\cdot \overline{q}\right)
DiracSlash[p, q]\left(\bar{\gamma }\cdot \overline{p}\right).\left(\bar{\gamma }\cdot \overline{q}\right)
DiracSlash is scheduled for removal in the future versions of FeynCalc. The safe alternative is to use GS.
GS[p]\bar{\gamma }\cdot \overline{p}
GSD[p]\gamma \cdot p
FCI[GS[p]] === DiracSlash[p]\text{True}
FCI[GSD[p]] === DiracSlash[p, Dimension -> D]\text{True}