They say ikkyo means something like "thing number one," but I often like looser, more colloquial translations, so I sometimes wonder if it might be better to think of "ikkyo" as "one thing," just like Curly the Cowboy in City Slickers.
There are many, many forms of ikkyo, or oshitaoshi that we practice. I think the idea is to look at this principle from lots of different angles and under the light of many different conditions, until it becomes one thing - ikkyo - in our minds instead of many things.
For me, the one thing that all these different forms seems to be congealing into is the idea of controlling uke's center by manipulating the primary cross (the spine and shoulder girdle) sort of like a rudder on a boat - but maybe you should play with these things for 10 or 15 years yourself and see what conclusions you can draw from ikkyo.
