Congratulations to Williams Sensei on his promotion to 10th dan in Aikido Yuishinkai by Master Koretoshi Maruyama. 
A long time student of Maruyama Sensei, Williams Sensei joined Aikido Yuishinkai just under a decade ago when Maruyama Sensei emerged form a period of seclusion in a temple. Since that time with Williams Sensei as the International chief instructor of the organisation it has continued to grow nationally and internationally, today it has around 100 dojo world wide. A key to its success has been not only the personal reputation and technical direction of Maruyama sensei, an uchi-deshi of the founder, but also the annual international seminar programme, a robust aikido syllabus for teaching, student guide(s) and DVD resources. All of this has been developed under a banner of 'aikido without boundaries' which has allowed many dojo internationally to reconnect with a living lineage to the founder of aikido under an organisational yoke that it light. Williams Sensei, together with a technical mastery of the art has played a vital role in the development of all of these as well as facilitating the err to the west and its wonderful to see him recognised for this.
These days in the martial arts we see more than a few grandmasters, 10th dan's and founders in the west, often self proclaimed, but very few emerge with living connections to Japan.
At the dojo people sometimes ask what does a dan grade mean? The meanings that count the most I suspect though are to the person receiving the award, the giver of the award and the school within it it is awarded. Beyond this there are no shortage of opinions. Aikido Historian Stanley Pranin has a bit to say on the topic  which is a thought provoking read.
Many compare dan rankings between one art and the other, or one school and another, for us in aikido its widely understood that a black belt is a qualified beginner where as in something like Brazillian JuJitsu it can take well over a decade to achieve and represents a high degree of skill and probably competitive success as well. Within the schools of aikido there is also great variation in dan rankings, 3-5 yrs. is a typical time frame in the west, but its often much quicker in Japan where its both less and more important and maybe more about a student that is committed to the dojo.