To fasten, to join to (literally and figuratively).

"An officer is attached to a certain regiment, company, or ship."

To adhere; to be attached.

To come into legal operation in connection with anything; to vest.

"Dower will attach."

To win the heart of; to connect by ties of love or self-interest; to attract; to fasten or bind by moral influence; with to.

"attached to a friend; attaching others to us by wealth or flattery"

To connect, in a figurative sense; to ascribe or attribute; to affix; with to.

"to attach great importance to a particular circumstance"

To take, seize, or lay hold of.

To arrest, seize.

Connected; joined.

Fond of (used with to).

"I'm very attached to my pets."

In a romantic or sexual relationship.

"As far as I know, he isn't attached, so I'm going to invite him out on a date."

Broadly joined to a stem or stipe, but not decurrent.

"In this group of mushrooms, the attachment of the gills to the stipe ranges from attached to almost decurrent."

Of a residential building, sharing walls with similar buildings on two, usually opposite, sides.