top (table of processes) package on Ubuntu 16.04 LTS

It is probably installed with the procps package. It is also common in all *NIX OSes. If you really need to find what it is installed with, install apt-file first:

sudo apt install apt-file && apt-file update

If you run a which command it will return the folder that it is installed into:

$ which top
/usr/bin/top

Then do a search like this:

apt-file search --regexp '/top$'

It will return a list like this:

broctl: /usr/share/broctl/scripts/helpers/top
crossfire-maps: /usr/share/games/crossfire/maps/santo_dominion/magara/well/top
crossfire-maps-small: /usr/share/games/crossfire/maps/santo_dominion/magara/well/top
liece: /usr/share/emacs/site-lisp/liece/styles/top
lubuntu-extra-sessions: /usr/share/lxpanel/profile/Lubuntu-Nexus7/panels/top
lxpanel: /etc/xdg/lxpanel/two_panels/panels/top
procps: /usr/bin/top
quilt: /usr/share/quilt/top
trn4: /usr/share/doc/trn4/examples/univ/top
trn4: /usr/share/trn4/HelpFiles/top

Then all you have to do is match the folder, and that is the package at the beginning of the line.

Or, you could combine the statement as one and it should return a more exact result:

$ apt-file search --regexp "^$(which top)$"
procps: /usr/bin/top

Hope this helps!

If you are using http://packages.ubuntu.com/ to find packages that provide top, you need to search for top in the "Search the contents of packages" section. The results of such a search are:

File                                                            Packages
/etc/xdg/lxpanel/two_panels/panels/top                          lxpanel
/usr/bin/top                                                    procps
/usr/share/broctl/scripts/helpers/top                           broctl
/usr/share/doc/trn4/examples/univ/top                           trn4
/usr/share/emacs/site-lisp/liece/styles/top                     liece
/usr/share/games/crossfire/maps/santo_dominion/magara/well/top  crossfire-maps, crossfire-maps-small
/usr/share/lxpanel/profile/Lubuntu-Nexus7/panels/top            lubuntu-extra-sessions
/usr/share/quilt/top                                            quilt
/usr/share/trn4/HelpFiles/top                                   trn4

In your case, you are most likely looking for the package that provides /usr/bin/top, which is procps.

Unsurprisingly, the search results are identical to the ones found using apt-file as mentioned in @Terrance's answer.