At first glance, the answer to this question is simple. Single-session therapy (SST) is therapy that lasts for one session. And indeed, there are times when therapy does last for a single session. For example, there are occasions when the therapist and the client contract for a 'block' of therapy sessions, but the client only attends the first session. This is known as 'single-session therapy by default'. There are also occasions when, for whatever reason, the client only wants to come for one session and only attends that session. Here the therapist and the contract both contract for that one session. When this occurs, this is known as 'single-session therapy by design'. Also, when I do a demonstration of single-session therapy at a training workshop, for example, then the volunteer knows that that session will be the only session that they will have with me.
However, the majority opinion in the SST community is that single-session therapy is 'an intentional endeavour where the client and therapist agree to meet to help the client deal with their nominated concern in one session knowing that more help is available if needed'. In this definition, several important points are made:
1. The work has a purpose. The intention is ideally to help the person in one session.
2. The client will get the help needed. Further therapy sessions are available if the client needs such help.
3. The work is consensual. The therapist and client agree to embark based on the first two points
There is also a view in the SST community that the client can be encouraged to prepare for this session and that this counts as a therapeutic contact. Let's also consider that most agencies which offer SST will follow up with the client to see how they are doing after the single-session. We are faced with the interesting conundrum that single-session therapy may involve three points of contact between the client and the therapist (or the agency where the therapist works) and may involve the client having further sessions.
Thus, what has started as a simple situation - SST is therapy that lasts for one session - has ended with the complicated situation where SST may comprise several therapeutic contacts and the possibility of further treatment following the 'single session' if the client needs it.
Given this complexity, why continue to use the term, 'single-session therapy'? Jeff Young (2018), an Australian SST therapist, argues for its retention because of its 'shock value' despite its ambiguities. The term fascinates professionals, challenges their assumptions and stimulates them to think through several issues concerning therapy length, client preferences and related points.
I agree with Young that the term, 'single-session therapy' should be retained as it reflects the following:
a. It is a way of practising therapy based on a way of thinking about therapy known as 'single-session thinking' which emphasises the intent of the work: to help the client in one session
b. It is a way of organising therapy services that caters to a significant number of clients who choose to attend for one therapy session. Thus, numerous reports show that the modal number of sessions clients have across settings and countries is 'one', followed by 'two' followed by 'three', etc. (e.g. Brown & Jones, 2005; Hoyt & Talmon, 2014).
In conclusion, when using the phrase 'single-session therapy', it is vital that
advocates of this way of working clarify how the term is being used and appreciate that it can be easily misunderstood.  The ‘mode’ is the most frequently occurring number in a series.
Brown, G.S., & Jones, E.R. (2005). Implementation of a feedback system in a managed care environment: What are patients teaching us? Journal of Clinical Psychology, 61, 187-198.
Hoyt, M.F., & Talmon, M.F. (2014). What the literature says: An annotated bibliography. In M.F. Hoyt & M. Talmon (Eds.), Capturing the Moment:
Single Session Therapy and Walk-In Services (pp. 487-516). Bethel, CT: Crown
Young, J. (2018). SST: The misunderstood gift that keeps on giving. In M.F. Hoyt, M. Bobele, A. Slive, J. Young, J., & M. Talmon, (Eds.), Single-Session Therapy by Walk-In or Appointment: Administrative, Clinical, and Supervisory Aspects of One-at-a Time Services (pp. 40-58). New York: Routledge.