To be a geometric sequence there must be a common ratio which basically means the number you multiply a term by to get the next one, and the common ratio must be the same between all terms. The only sequence that follows this is the first one.
Each term of a geometric sequence is a multiple of the previous term. For example, if the first term is 2 and the common ratio is 3, then the next term is 6; the next term is 18. And so on.
1,2,4,8,16.... fits this pattern. The first term is 1 and each succeeding term is 2 times the previous term: 1*2 = 2; 2*2 = 4; 4*2 = 8, and so on.
