secondly

As an adverb secondly

is in the second place.

As an adjective false is

(label) one of two states of a boolean variable; logic.

As adverbs the difference between secondly and secondly

is that secondly is in the second place while secondly is in the second place.

As an adverb secondly

is in the second place.

As an adjective undefined is

lacking a definition or value.

As adverbs the difference between twice and secondly

is that twice is two times while secondly is in the second place.

As adverbs the difference between firstly and secondly

is that firstly is (formal) in the first place; before anything else; first while secondly is in the second place.

As adverbs the difference between subsequently and secondly

is that subsequently is following, afterwards in either time or place while secondly is in the second place.

As adverbs the difference between again and secondly

is that again is (label) back in the reverse direction, or to an original starting point while secondly is in the second place.

As a preposition again

is (obsolete or dialectal) against.