Trendiness is the quality of being trendy — of following, or looking like it follows, whatever is fashionable right now. It is the noun for that quality, so you use it when you want to talk about the fashionableness itself rather than about the thing: 'the trendiness of the neighbourhood', 'the trendiness of these glasses'. Here is the part a dictionary often leaves out: trendiness is usually not a compliment. When people say 'trendy' about a café or a haircut, they often mean it is fashionable but shallow — it is doing what everyone is doing, and it will look silly in three years. So 'trendiness' very often carries a small criticism: 'I got tired of the trendiness of that area', 'the design is all trendiness and no substance'. It can be neutral, especially in business or marketing writing ('the trendiness of the brand attracted younger buyers'), but you should hear the doubt in it and use it knowing that doubt is there. Compare 'style' or 'stylishness', which suggest something that lasts, and 'popularity', which is just about numbers of people.

趕流行、跟風的（性質）；時髦感（常帶「只是跟風、沒內涵」的貶意）