Seal

The word seal occurs several times in the DWU:

As symbol associated with an office

 * Seal of Rassilon, a Time Lord seal of state of uncertain application (DW: The Invasion of Time, Doctor Who (1996), and others)
 * Great Seal of Diplos (DW: The Stones of Blood)

As locking mechanism

 * Deadlock seals, an effective barrier against sonic devices. (DW: School Reunion, The Time of Angels and others)
 * Hardinger seals, as used on Bowie Base One (DW: The Waters of Mars)

Animals

 * Seal (animal), Earth marine mammals, referenced in DW: Horror of Fang Rock