A reference to what is presumably a synonym
for a force field.
The term is used in two episodes
(verified against transcripts in The Catacombs).
Episodes
"The Metamorph":
This term is used by
Mentor
for a barrier which also reflects laser energy backwards
(and which killed Lew Picard).
Note: Maya uses the term "force field"
for other energy barriers.
They are presumably synonyms for the same and/or similar things.
See Force Fields on Psychon.