Update glossary.md - Prathread (#5891)

docs/general/glossary.md

@@ -462,9 +462,11 @@ A unique numeric (non-negative integer) identifier for a parachain.
 
 ## Parathread
 
-Historically, the term "parathread" has been used in two contexts: a substrate-based chain not yet
+Historically, the term "parathread" has been used in two contexts: a chain registered with a `ParaID` 
+on the [relay chain](#relay-chain) that has not yet become a
 parachain (i.e. not connected to a {{ polkadot: Polkadot :polkadot }}{{ kusama: Kusama :kusama }}
-core), or as an on-demand parachain (i.e. a parachain using on-demand coretime).
+core), or as an on-demand parachain (i.e. a parachain using on-demand coretime) which does not produce
+blocks at regular intervals.
 
 ## Parachain Registry