Definition
An embedded clause develops when one clause is embedded inside of another, instead of keeping the two on an equal level i.e., as separate statements. (Carnie, 2006, p. 47)
These clauses are termed relative clauses, subordinate clauses, and nominalized clauses. (Suttles, 2004, p. 94)
In Halkomelem, there are four clause-initial particles that introduce two kinds of embedded clauses – subordinate and nominalized. (Suttles, 2004, p. 38)
Examples

References
Carnie, A. (2006). Syntax: A generative introduction. John Wiley & Sons.
Suttles, W. (2004). Musqueum reference grammar. UBC Press. SFU Student Access.