It can be. But it is not necessarily the case:
Definition of invocation
1a: the act or process of petitioning for help or supportspecifically, often capitalized : a prayer of entreaty (as at the beginning of a service of worship)
b: a calling upon for authority or justification
2: a formula for conjuring :
INCANTATION
3: an act of legal or moral implementation :
ENFORCEMENT
Definition of INVOCATION
But even in the first definition it is not necessarily a prayer. And many theists use it as definition 2.