Mid cap stands for middle capitalisation and is a term used to group stocks and shares. Sitting between large- and small-cap stocks, mid-cap stocks used to have a valuation of between $1 and $5 billion but more recently are defined as having a valuation of between $2 and $10 billion.
Some stock index providers have mid-cap indices. For example, the FTSE 100 is a large-cap index, while the FTSE 250 is a mid-cap index and the FTSE Small Cap is as the name suggests a small-cap index.