Subthreshold is a term that is often used in various fields, from psychology to electronics. It refers to a level or state that is just below a threshold or limit. However, there are some synonyms that can be used in place of subthreshold, such as below the limit, under the threshold, or beneath the minimum. Additionally, some related terms could be subminimal, suboptimal, or substandard. These similar words can be used depending on the context, and can help to add variety to your writing or conversation. It's always useful to have a range of synonyms at your disposal to keep your language varied and interesting.