# GLPK: meaning of the "marginal' column in the solution output

I'm using GLPK to solve an LP.
I use it through its standalone solver, that I call with the glpsol command, and I get the solution detail written in a file using the -o/--output option.

I have no problem parsing the file to recover the actual solution, but I'm wondering what is the meaning of the "marginal" column.
Here is what the output file looks like:

Problem:
Rows:       60
Columns:    9821
Non-zeros:  405355
Status:     OPTIMAL
Objective:  obj = 1.355170551 (MINimum)

No.   Row name   St   Activity     Lower bound   Upper bound    Marginal
------ ------------ -- ------------- ------------- ------------- -------------
1 r.9825       B        1.07478             1
2 r.17470      NL             1             1                   0.0287012
3 r.24250      B        1.23575             1
4 r.31488      NL             1             1                       < eps
5 r.38796      B         1.0038             1
6 r.44423      B        1.03857             1
7 r.50832      NL             1             1                   0.0109038
8 r.57288      NL             1             1                   0.0233011
// some more rows ......

No. Column name  St   Activity     Lower bound   Upper bound    Marginal
------ ------------ -- ------------- ------------- ------------- -------------
1 Z1           NL             0             0                    0.233221
2 Z2           NL             0             0                    0.174953
3 Z3           NL             0             0                    0.157688
4 Z4           NL             0             0                    0.164336
5 Z5           NL             0             0                     0.20934
6 Z6           NL             0             0                    0.124799
[ .... ]
237 Z237         NL             0             0                   0.0686442
238 Z238         NL             0             0                    0.159257
239 Z239         B      0.0424075             0
240 Z240         NL             0             0                   0.0370097
241 Z241         NL             0             0                    0.324198
[ .... ]



The actual values I'm looking for are actually referred to as "activity" (my decision variables being the Zn).
But what does marginal represent? And, while I'm here, why are the solution values called activity (there must be a historical/theoretical reason, and I'm interested)?