What is Adhering?

Legal Definition
Cleaving to, or joining; as, adhering to the enemies of the United States.

2. The constitution of the United States, art. 3, s 3, defines treason against the United States, to consist only in levying war against them or in adhering to their enemies, giving them aid and comfort.

3. The fact that a citizen is cruising in an enemy's ship, with a design to capture or destroy American ships, would be an adhering to the enemies of the United States. 4 State Tr. 328 ; Salk. 634; 2 Gilb. Ev. by Lofft, 798.

4. If war be actually levied, that is, a body of men be actually assembled for the purpose of effecting by force a treasonable enterprise, all those who perform any part, however minute, or however remote from the scene of action, and who are leagued in the general conspiracy are to be considered as traitors. 4 Cranch. 126.
-- Bouviers Law Dictionary
Legal Definition
Joining, leagued with, cleaving to; as, "adhering to the enemies of the United States." Rebels, being citizens, are not "enemies," within the meaning of the constitution ; hence a conviction for treason, in promoting a rebellion, cannot be sustained under that branch of the constitutional definition which speaks of "adhering to their enemies, giving them aid and comfort." United States v. Greathouse, 2 Abb. (U. S.) 364, Fed. Cas. No. 15.254.
-- Black's Law Dictionary